Calystegia soldanella (Convolvulaceae) is a self-incompatible perennial herb distributed on sandy seashores throughout the temperate zone of the world. In Japan, the species occasionally grows on the sandy shores of Lake Biwa. To clarify the genetic differentiation among local populations, we investigated the nucleotide sequence variability of the Adh gene. In a 1625-bp sequence between exon 2 and the 3' noncoding region of the Adh gene, a total of 44 polymorphic sites were found among 91 individuals from 19 populations. The nucleotide diversity for the entire sample was 0.00212. Similar values were determined for geographical groups of populations. No genetic differentiation among the groups of populations was found. The complete lack of genetic differentiation between the sea coastal populations and the inland populations could not be attributed to gene flow. Although the inland populations are geographically isolated from the sea coastal populations, the time since separation might be insufficient to establish significant genetic differentiation.