The population of filth flies attracted to fish was studied at the University of Michigan Biological Station in Cheboygan County, Michigan, during the summer session of 1954. At least 48 species were captured. The percent of the total fly population represented by each species (in some instances by each genus), the population peak, and the choice between kitchen and beach as a local habitat was determined for each species and presented in table 1. About 84 percent of the total fly population was captured more frequently at the beach than at the kitchen. Biological differences in habitat and seasonal prevalence between Muscina assimilis and M. stabulans are brought to light. The house fly was not as important a pest as were other species during the summer session of 1954.