The Comal River, a spring-fed system in central Texas, USA, was invaded in the 1960s by two Asian aquatic snails (Thiaridae: Melanoides tuberculata and Tarebia granifera) and, subsequently, three of their trematode parasites (Centrocestus formosanus 1990s, Haplorchis pumilio 2000s, and Philophthalmus gralli 1960s). Previous snail collections (2001-2002) established that habitat conditions significantly affect the distribution of both snail species. However, the effect of snail size (known to influence infection prevalence), and habitat conditions (known to influence snail size), on trematode infection patterns in this system was not evaluated. In a re-evaluation of this dataset, logistic regression analyses by individual snail showed that for both M. tuberculata and T. granifera populations, large snails were more likely to be infected than small snails and habitat conditions were significantly related to infection in T. granifera. However, only snail size was significant in explaining the probability of infection in M. tuberculata. This result was confirmed by linear regression models that showed that both infected and non-infected M. tuberculata used similar habitats as large individuals in both infection categories were found in patches dominated by fine substrates and high levels of aquatic vegetation and detritus. With large size class T. granifera, non-infected individuals were found primarily in habitats with silt/sand substrates and high vegetation and detritus cover while infected individuals occurred among all available habitats. Using these results, the authors suggest targeted sampling of large individuals of M. tuberculata in habitats with high detritus and vegetation and large individuals of T. granifera in any habitat can be used to efficiently ascertain parasite “hotspots” and evaluate changes in parasite prevalence or detect the invasion of new parasites in these thiarid snails.