A survey of the helminth fauna of largemouth bass (Micropterus salmoides) was conducted to examine helminth biodiversity and community structure in a northwestern Missouri reservoir. Twenty bass were collected from Smithville Reservoir in May and again in September of 1996 and necropsied in search of helminth parasites. Seven species of helminths were recovered:Proteocephalus ambloplitis, Spinitectus sp., Contracaecum sp., Camallanus sp., Posthodiplostomum minimum, Crepidostomum sp., and Neoechinorhynchus cylindratus. The acanthocephalan,N. cylindratus, was the most prevalent parasite in fish sampled in May with 95% of the fish infected with this species. The plerocercoid ofP. ambloplitiswas the most prevalent parasite in fish sampled in September, again with 95% of the fish infected. For both of these species, the high prevalence in one month was significantly different from its prevalence in the other month. When any of the seven parasite species was significantly more prevalent in a given month, it also comprised a significantly larger portion of the helminth community and exhibited a significantly greater mean abundance. The frequency distribution of the number of parasite species per host was similar between months, but the parasite species diversity was significantly different with the component community of May being more diverse than that of September.