Exotic poeciliid fishes introduced into Hawaiian freshwaters are responsible for the introduction of several exotic parasites, of which the most important are Camallanus cotti and Bothriocephalus acheilognathi in terms of potential disease threat to native stream fishes. This roundworm and tapeworm are the most prevalent and abundant freshwater fish helminths in Hawaiian streams. This study examined the seasonal and yearly population structure of C. cotti and B. acheilognathi to determine if the tropical Hawaiian environment characterized by low climatic variability permits continuous opportunities for parasite transmission regardless of time of year. Camallanus cotti displayed seasonal differences in prevalence and mean abundance, whereas B. acheilognathi did not. Camallanus cotti prevalence and mean abundance were higher in the Hawaiian summer (47.7%, 0.79) than in winter (25.8%, 0.36). A seasonal relationship of C. cotti levels is likely explained by extensive rains associated with the Hawaiian winter season, which may act to decrease parasite transmission by flushing infected poeciliid hosts, intermediate copepod hosts, and possibly free-living infective worm stages downstream. Bothriocephalus acheilognathi displayed low prevalence and mean abundance in both summer (4.0%, 0.06) and winter (6.5%, 0.07), and it may be difficult to detect seasonal changes due to these low levels. Camallanus cotti prevalence and mean abundance remained relatively constant from the summer of 1995 to the summer of 1999, indicating that levels of this roundworm are stable in Waianu Stream. Whereas B. acheilognathi prevalence and mean abundance were low during the summer of 1995 and the summer of 1997, a dramatic peak in prevalence and mean abundance was observed in the summer of 1998 (41.2%, 1.06), with levels decreasing sharply in the summer of 1999 (4.4%, 0.07). It appears that B. acheilognathi also is present in stable populations at low levels, even though levels rose sharply during a single year.