diff --git a/souporcell_pipeline.py b/souporcell_pipeline.py
index f001574..0e3fe73 100755
--- a/souporcell_pipeline.py
+++ b/souporcell_pipeline.py
@@ -145,17 +145,22 @@ def get_fasta_regions(fastaname, threads):
     return regions
 
 
-def get_bam_regions(bamname, threads):
+def get_bam_regions(bamname, threads, known_chroms=None):
     bam = pysam.AlignmentFile(bamname)
+    refs = list(bam.references)
+    if known_chroms is not None:
+        refs = [r for r in refs if r in known_chroms]
+        assert len(refs) > 0, "No BAM references matched the chromosomes from the provided VCF(s)."
+    
     total_reference_length = 0
-    for chrom in bam.references:
+    for chrom in refs:
         total_reference_length += bam.get_reference_length(chrom)
     step_length = int(math.ceil(total_reference_length / threads))
     regions = []
     region = []
     region_so_far = 0
     chrom_so_far = 0
-    for chrom in bam.references:
+    for chrom in refs:
         chrom_length = bam.get_reference_length(chrom)
         #print(chrom+" size "+str(chrom_length)+" and step size "+str(step_length))
         while True:
@@ -180,6 +185,26 @@ def get_bam_regions(bamname, threads):
     
     return regions
 
+def read_vcf_chroms(paths):
+    """
+    Return a set of chromosome names seen in one or more VCF files.
+    Only inspects body lines (ignores headers entirely).
+    """
+    if isinstance(paths, str):
+        paths = [paths]
+
+    contigs = set()
+
+    for path in paths:
+        with open_function(path) as fh:
+            for line in fh:
+                if line.startswith("#"):
+                    continue
+                chrom = line.split("\t", 1)[0]
+                contigs.add(chrom)
+    
+    return contigs
+
 def make_fastqs(args):
     if not os.path.isfile(args.bam + ".bai"):
         print("no bam index found, creating")
@@ -350,9 +375,12 @@ def freebayes(args, bam, fasta):
         if not(args.known_genotypes == None):
             print("using known genotypes")
             args.common_variants = args.known_genotypes
+
+        known_chroms = read_vcf_chroms(args.common_variants)
+        print(f"Restricting BAM regions to {len(known_chroms)} chromosomes from provided VCF(s).")
         
         # parallelize the samtools depth call. It takes too long
-        regions = get_bam_regions(bam, int(args.threads))
+        regions = get_bam_regions(bam, int(args.threads), known_chroms=known_chroms)
         depth_files = []
         depth_procs = []
         print(len(regions))