Recently, some authors (Kennedy, 1981; Price & Clancy, 1983) have argued that there are fundamental differences between the communities of helminths in fish and bird hosts. Such differences are foreshadowed by the work of Dogiel (1964) and are apparent from survey data (e.g. Threlfall, 1967; Bakke, 1972; Hair & Holmes, 1975 on birds, and compare Chubb, 1963; Mishra & Chubb, 1969; Wootten, 1973; Ingham & Dronen, 1980 on fish). Questions still remain, however, as to whether the distinctions are truly justified and whether the differences are really fundamental. In this paper, we address these questions by examining helminth diversity in a series of hosts. We then discuss and provide explanations for the observed differences.