Because of the potentially important role of parasite excretory-secretory (ES) products in regulating intermate relationships and host-parasite interactions, we investigated intraspecific variability of adult ES products from a trematode species. Adults from 3 geographical isolates of Echinostoma caproni were collected and maintained under in vitro conditions. ES products were collected at 4, 8, and 22 hr of in vitro maintenance. In order to test for interspecific variability, ES products from a different echinostome species (Echinostoma sp.) were collected in a similar way. Major ES polypeptides were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation and silver staining. The polypeptide patterns of total ES products appeared very stable in time for each isolate. ES products from Echinostoma sp. showed a distinct polypeptide profile with none of the major bands being shared with the E. caproni isolates. Although polypeptide patterns from the 3 E. caproni isolates shared most major bands, isolate-specific bands could be observed. Two isolates exhibited a band at 85 and 119 kDa, respectively, whereas the third isolate was characterized by the absence of both bands. These results, together with the previously reported role of ES products in molecular signaling raise the question of the importance of intraspecific ES product differences in evolutionary processes such as assortative mating or local host adaptation.