diff --git a/reduction_tools/select_xtals.py b/reduction_tools/select_xtals.py
new file mode 100644
index 0000000..58a75ed
--- /dev/null
+++ b/reduction_tools/select_xtals.py
@@ -0,0 +1,215 @@
+#!/usr/bin/env python3
+
+# author J.Beale
+
+"""
+# aim
+to select crystals from a stream file based on the uc distances and angles
+based on a tolerance to them
+
+# usage
+python select_crystals.py -s <input stream file>
+                          -o <ouput file name>
+                          -c <list of cell lengths, comma separted, no spaces>
+                          -a <list of angles, comma separated, no spaces>
+                          -t <tolerance on included crystals>
+
+# output
+.stream file with extracted crystals based on tolerance of uc
+"""
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "-s",
+        "--stream",
+        help="input stream file",
+        required=True,
+        type=str,
+    )
+    parser.add_argument(
+        "-o",
+        "--output",
+        help="output stream file with selected crystals",
+        type=str,
+        default="selected_crystals.stream"
+    )
+    parser.add_argument(
+        "-c",
+        "--cell",
+        help="cell lengths to be found in A, e.g. 73.4,68.7,75.6 - must be a float/no spaces",
+        type=list_of_floats,
+        required=True
+    )
+    parser.add_argument(
+        "-a",
+        "--angles",
+        help="anlges to be found, e.g., 90.0,105.6,90.0 - must be a float/no spaces",
+        type=list_of_floats,
+        required=True
+    )
+    parser.add_argument(
+        "-t",
+        "--tolerance",
+        help="tolerance for angles and cell lengths",
+        type=float,
+        default=0.25
+    )
+
+
+# modules
+import re
+import argparse
+
+# Function to check if cell parameters are within the specified range
+def is_within_range( params, target_cell_params, target_angles, tolerance ):
+    for i in range(3):
+        if abs(params[i] - target_cell_params[i]) > tolerance:
+            return False
+    for i in range(3, 6):
+        if abs(params[i] - target_angles[i - 3]) > tolerance:
+            return False
+    return True
+
+def extract_chunks( input_file, target_cell_params, target_angles, tolerance ):
+
+    # setup
+    chunks = []
+    collect_lines = False
+
+    # Open the input file for reading
+    with open(input_file, 'r') as f:
+        for line in f:
+            # Check for the start condition
+            if line.strip() == '----- Begin chunk -----':
+                collect_lines = True
+                target_chunk = False
+                chunk_lines = []
+            # Collect lines between start and end conditions
+            if collect_lines:
+                chunk_lines.append(line)
+            if line.startswith('Cell parameters'):
+                match = re.search(r"(\d+\.\d+)\s(\d+\.\d+)\s(\d+\.\d+)\snm,\s(\d+\.\d+)\s(\d+\.\d+)\s(\d+\.\d+)\sdeg", line)
+                if match:
+                    params = [float(match.group(i)) for i in range(1, 7)]
+                    if is_within_range( params, target_cell_params, target_angles, tolerance ) == True:
+                        target_chunk = True
+            # Check for the end condition
+            if line.strip() == '----- End chunk -----':
+                collect_lines = False  # Stop collecting lines
+                if target_chunk == True:
+                    chunks.append( chunk_lines )
+    
+    return chunks
+
+def get_header( header, input_file ):
+
+    if header == "geom":
+        start_keyword = "----- Begin geometry file -----"
+        end_keyword = "----- End geometry file -----"
+    if header == "cell":
+        start_keyword = "----- Begin unit cell -----"
+        end_keyword = "----- End unit cell -----"
+
+    # setup
+    collect_lines = False
+    headers = []
+
+    # Open the input file for reading
+    with open(input_file, 'r') as f:
+        for line in f:
+            # Check for the start condition
+            if line.strip() == start_keyword:
+                collect_lines = True
+                headers_lines = []
+            # Collect lines between start and end conditions
+            if collect_lines:
+                headers_lines.append(line)
+            # Check for the end condition
+            if line.strip() == end_keyword:
+                collect_lines = False  # Stop collecting lines
+                headers.append(headers_lines)
+    
+    return headers[0]
+
+def write_to_file( geom, cell, sections, output_file ):
+
+    # Write sections with matching cell parameters to the output file
+    with open(output_file, 'w') as out_file:
+        out_file.write('CrystFEL stream format 2.3\n')
+        out_file.write('Generated by CrystFEL 0.10.2\n')
+        out_file.writelines(geom)
+        out_file.writelines(cell)
+        for section in sections:
+            out_file.write('--- Begin crystal\n')
+            # out_file.writelines('--- Begin crystal\n' + section + "\n--- End crystal")
+            out_file.writelines(section)
+            out_file.write('--- End crystal\n')
+            out_file.write('----- End chunk -----\n')
+
+def main( input_file, output_file, target_cell_params, target_angles, tolerance):
+
+    # get geom and cell file headers
+    print( "getting header info from .stream file" )
+    geom = get_header( "geom", input_file )
+    cell = get_header( "cell", input_file )
+    print( "done" )
+    
+    # get chunks with defined uc tolerances
+    print( "finding crystals with unit cells of around:" )
+    print( "{0} nm".format( target_cell_params ) )
+    print( "{0} deg".format( target_angles ) )
+    chunks = extract_chunks( input_file, target_cell_params, target_angles, tolerance )
+    print( "found {0} crystals".format( len(chunks) ) )
+
+    # write to file
+    print( "writing to new file" )
+    write_to_file( geom, cell, chunks, output_file )
+    print( "done" )
+
+def list_of_floats(arg):
+    return list(map(float, arg.split(',')))
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "-s",
+        "--stream",
+        help="input stream file",
+        required=True,
+        type=str,
+    )
+    parser.add_argument(
+        "-o",
+        "--output",
+        help="output stream file with selected crystals",
+        type=str,
+        default="selected_crystals.stream"
+    )
+    parser.add_argument(
+        "-c",
+        "--cell",
+        help="cell lengths to be found in A, e.g. 73.4,68.7,75.6 - must be a float/no spaces",
+        type=list_of_floats,
+        required=True
+    )
+    parser.add_argument(
+        "-a",
+        "--angles",
+        help="anlges to be found, e.g., 90.0,105.6,90.0 - must be a float/no spaces",
+        type=list_of_floats,
+        required=True
+    )
+    parser.add_argument(
+        "-t",
+        "--tolerance",
+        help="tolerance for angles and cell lengths",
+        type=float,
+        default=0.25
+    )
+    args = parser.parse_args()
+    # convert cell into nm
+    cell = [ round( x/10,4 ) for x in args.cell ]
+    # run main
+    main( args.stream, args.output, cell, args.angles, args.tolerance )
+
+