An important issue in intraspecific molecular phylogenetic studies concerns distribution of genetic variation within and among populations and, hence, within-population sample sizes used in analyses. To address this sampling issue, we sequenced a 795 base pair (bp) segment of the mitochondrial cytochrome b gene from 19 unrelated individuals from a Louisiana population of the smooth softshell turtle (Apalone mutica LeSueur). We found a complete lack of within-population variation in this large segment of mtDNA. This result supports the use of minimal within-population sample sizes in intraspecific molecular phylogenetic studies of Apalone using cytochrome b.