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Salsuginus yutanensis occurs on the gills of the Plains topminnow Fundulus sciadicus Cope. The fish of this species have been found to vary morphologically and biochemically among disjunct populations. Morphological characteristics of the sclerotized parts of S. yutanensis were examined from three localities in Nebraska, over a 2-yr collecting period. Analysis of variance was used to assess morphological variation with respect to site and date. Worms from two localities, Keith and Saunders counties, differed significantly for most characters considered. A third site, also in Keith County, contained worms for which measurement means tended to be intermediate between those in the other two sites. This site-related difference was maintained over a pattern of broad seasonal variation and suggests that the site-related differences are of evolutionary origin. If this interpretation is true, then the parasite populations likely are isolated in a manner analogous to those of the host. However, differences due to effects of temperature on worm development were not ruled out as possible explanations for the observations although consistent temperature differences between the sites are unlikely, given the nature of the habitats studied.