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Present theory states that in species-poor assemblages interactions among species are negligible and assemblage structure is a result of host/parasite interactions. A corollary to this theory is that parasite species assemblages should behave as accumulations of multiple-kind items drawn from a population in which only the relative probability of drawing each kind varies. This study was designed to test this prediction by comparing descriptor values, including diversity indices, species density values, equitability, and prevalence to those obtained from data produced by a Monte Carlo simulation model. The parasite assemblage consisted of 7 parasite species or types found in the fathead minnow, Pimephales promelas. The data presented indicate that all of the assemblage descriptors are positively intercorrelated, as in the Monte Carlo simulations. Also, the field data are described best by a model in which a single population structuring mechanism operates, suggesting that aggregating mechanisms are environmental in origin and act on all parasite populations more or less uniformly. In addition, the P. promelas assemblage structure appears to be a function of current year streamflow, suggesting that abiotic factors influence parasite assemblage structure through effects on individual parasite species' probability of infection.