Throughout streams and rivers in Michigan, populations of benthic invertebrates can be regulated by microsporidian parasites. For example, studies in the literature demonstrate that population decreases of the herbivore caddisfly Glossosoma nigrior by the parasite Cougourdella sp. result in a shift of the invertebrate community and an increase in periphyton biomass. Similarly, studies on the Kalamazoo River in southern Michigan indicate that the filter feeder, Brachycentrus americanus, may also be regulated by a microsporidian parasite Pyrotheca sp. In addition, the deposit feeder, Gammarus pseudolimnaeus is also routinely infected by several microsporidian parasites. However, it is unknown how widespread microsporidian infections are among B. americanus and G. pseudolimnaeus in Michigan. The main objective of my study was to estimate the prevalence of microsporidian infection for these two taxa throughout Michigan streams. Benthic invertebrates were collected during May of 1994 from 64 trout streams located in the lower peninsula of Michigan. Individuals of B. americanus and G. pseudolimnaeus were examined for the presence of infection. B. americanus was present at 26 of the 64 sites examined, and Pyrotheca infections were detected at only one site (4% infection rate among individuals at this site). Gammarus pseudolimnaeus was present at 66% of the sites; 31% of those sites harbored microsporidian infections. These data can be compared with results obtained from a parallel study of Glossosoma nigrior. Glossosoma nigrior was present in 53 of the 64 sites, and 72% of those sites had infected individuals. This comparison revelaed no statistical evidence that the presence of infected Glossosoma nigrior is related to the presence of infected G. pseudolimnaeus among the 64 sites studied. Age class and gender analyses indicate that older individuals are more susceptible to infection than younger individuals for both B. americanus and G. pseudolimnaeus. Although there is a significant difference in G. pseudolimnaeus infection rates among males, females, immatures and unknowns combined, there was no significant difference found when comparing males and females only. Because B. americanus, G. pseudolimnaeus and G. nigrior are often important contributors to the food web dynamics of many Michigan streams, knowledge of the effect of parasite-host interactions on invertebrate densities is essential to a clear understanding of trout stream community dynamics.