Introduction of exotic fishes into Hawai'ian streams has resulted in the simultaneous introduction of exotic parasites. Camallanus cotti (Nematoda) and Bothriocephalus acheilognathi (Cestoda) are the most prevalent and abundant fish helminths in Hawai'ian streams. The population structure and host specificity of C. cotti and B. acheilognathi in exotic poeciliids were examined during May and June 1995 in Waianu Stream, O‘ahu, Hawai'i. Prevalence and mean abundance of C. cotti were significantly different among Poecilia reticulata (65.2%, 1.05), Poecilia mexicana (49.0%, 0.87), and Xiphophorus helleri (32.3%, 0.44). Prevalence of B. acheilognathi was significantly higher in P. mexicana (6.1%) than in P. reticulata (2.1%) and X. helleri (1.6%). However, tapeworm differences in mean abundance were not significant among P. mexicana (0.08), P. reticulata (0.04), or X. helleri (0.03). Nematode and tapeworm prevalence and mean abundance were not significantly different with regard to fish sex. Camallanus cotti prevalence and mean abundance increased as P. mexicana body size increased (r² = 0.84 and r² = 0.72, respectively), whereas B. acheilognathi displayed no significant trend with respect to poeciliid body size.