During a routine paternity casework, performed with automated genotyping using an AmpFISTR Identifiler kit, an inconsistency affecting maternal segregation of D13S317 allele was encountered, manually detected as a variant allele in the mother and child. Alleles of the putative father were transmitted in 13 out of 15 autosomal STR loci, but in CSF1PO locus, there was an apparent mutation. We, therefore, directly sequenced the variant D13S317 allele in the mother and the child and compared the results to the available data on variant alleles within this STR locus. The variant allele consisted of 6 TATC repeats and an additional AATC motif, thus, by a similarity to the previously reported variant, it was labeled D13S317.6'. It seems that the variant allele is quite rare in the Polish population, however, its electrophoretic mobility between preceding TH01 alleles and that of D13S317 one, requires a careful scrutiny of automated genotyping traces to avoid misinterpretation of the results.