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The population dynamics of 7 parasite species or ecotypes were studied in the plains killifish, Fundulus zebrinus, at a single collection site in the South Platte River of Nebraska, U.S.A., for 5 yr. Parasites were: Myxosoma funduli (gill), Trichodina sp. (gill), Gyrodactylus bulbacanthus (gill), Urocleidus fundulus (gill), Gyrodactylus stableri (body surface), and Neascus sp. (= Posthodiplostomum; eyes and body cavity). Parasite densities, prevalences, and variance/mean ratios are reported for 22 samples, totaling 447 fish, over the summer months. Positive correlations were obtained between density and both standardized prevalence and log-transformed variance. Urocleidus fundulus populations were relatively stable with low densities and low variance/mean ratios. Most other species' populations exhibited disproportionate increases in aggregation with increases in density. The Neascus species showed long-term decreases in density and prevalence associated with increases in river flow rates.