We used microsatellite analysis to investigate possible reproduction by parthenogenesis in the bisexual Sierra garter snake, Thamnophis couchii. The genotypes of four microsatellite loci were determined for each of four individuals, including three T. couchii, two of which were a female and her offspring, and T. ordinoides. The female T. couchii and her offspring were homozygous and identical for all four microsatellite loci; however, Ts3 and Ts4 are the only loci that supported reproduction by parthenogenesis as Ts1 and Ts2 were uninformative. These data along with absence of a unique paternal allele and known absence of an opportunity to mate prior to and during captivity is consistent with reproduction by parthenogenesis.