CSF1PO is one of the thirteen core loci used for the CODIS database, and alleles reported for this short tandem repeat (STR) locus contain from 6 to 15 repeats of the tetranucleotide AGAT. Screening of DNA from 76 individuals by gel electrophoresis and silver stain detection yielded one sample that contained a rare, off-ladder CSF1PO allele; an allele larger than CSF1PO15 was detected in a heterozygote that also contained a CSF1PO10 allele. Capillary electrophoresis analysis using GeneScan software demonstrated that the variant allele contained four bases more than CSF1PO15. Following agarose gel electrophoresis to separate the two alleles of the heterozygote and cycle sequencing using dye terminators, sequence analysis showed that the variant, which was otherwise identical to the CSF1PO GenBank sequence, contained exactly 16 AGAT repeats. These results demonstrate the existence of an additional CSF1PO allele, a previously unreported size variant, CSF1PO16.