A BASIC computer simulation model was constructed to mimic the dynamic behavior of an assemblage of parasite species over a range of abiotic conditions. Computer hosts sampled a parasite supra-assemblage consisting of seven parasite types which differed in their relative probabilities of infection. In this model, it was possible for the hosts to unsuccessfully sample the supra-assemblage, as must happen in nature. Parasite population aggregation, competition, and co-occurrence were also simulated. A single simulation run produced a sample of 24 host individuals with their respective parasite assemblages. Species density, mean infra-assemblage diversity, sample assemblage diversity, and mean prevalence were used as descriptors of assemblage structure. The model data were compared to a field data set consisting of 33 samples of the fish, Fundulus zebrinus, and its parasites, taken during a 7 year period from a single location in a fluctuating river. The results suggested that over the long term, field data did not depart significantly from that predicted by a null model. Results also suggested that if aggregation and co-occurrence operated to influence assemblage structure, then the correlation between sample assemblage diversity values, and those of the other descriptors (species density, infra-assemblage diversity, and mean prevalence) should be reduced. Competition had no effect on the long term dynamics of the computer assemblage.