An isolated population of Unionicola fossulata (Wolcott) in one Michigan lake averaged 1. 96 [plus or minus] 0. 08 mites per mussel and infected about 90% of the host, Lampsilis siliquoidea (Barnes). Incidence and total numbers of mites were constant during the time when adult mites died and were replaced by invading mite larvae indicating that the number and sex of invading larvae must very nearly balance the deaths of adult mites. The location of mites in the host shows that ovipositing females or their eggs tend to mark out 2 exclusive territories in each host. Consequently most parasitized hosts carry only 1 or 2 female mites plus 1 male. It is argued that specialized oviposition behavior may be advantageous because of the increased hatching of precisely placed eggs and that this restricts mites to specific sites which can be filled with eggs from one female. Excess females may reduce, or at least not increase, the hatch of eggs; thus, any behavior that tends to exclude excess females would be selectively favored. This evolutionary trend, not interspecific competition, may have forced Unionicola species to lower population densities and restrict activity in the host, thus permitting the development of sympatry.