The topography of Thailand is characterized by low-lying agricultural lands abruptly punctuated by discontinuous mountain ranges, with their associated streams and aquatic insect communities. Common members of these communities are water measurers of the genus Hydrometra (Heteroptera: Hydrometridae), of which 12 species are known from Thailand. The similarity of Hydrometra communities is examined among isolated mountain ranges and between lowlands north and south of the Isthmus of Kra. Concordance in biogeographic patterns between mountain ranges is evaluated using two-way indicator species analysis (TWINSPAN) and cluster analysis. Differences in species richness among mountain ranges or between lowland regions is evaluated using separate one-way analyses of variance (ANOVA). Distributions are given for the 12 species recorded from Thailand, including for a new species recently discovered from Kanchanaburi Province in western Thailand.