Certain factors controlling the parasitic phase of some nonspecific fish ectoparasites were investigated. Studies were made of the infection patterns exhibited by the branchiuran Argulus foliaceus (L. ), the fish leeches Piscicola geometra (L. ) and Hemiclepsis marginata (Mull. ) and glochidia larvae of Anodonta cygnea (L. ) on freshwater fish. Aspects of the parasites life history, survival, development, seasonal cycles in incidence, dispersion patterns, immigration and site selection were investigated by field and laboratory studies. In adult A. foli. aceus and P. geometra, the initial attacking a phase onto the fish host does not appear to be influenced by densitydependent factors. In larval A. foliaceus, infection is also unaffected by parasite density, but in adults, it is controlled at high parasite numbers when the carrying capacity of the fish is reached. The infection patterns of A. foliaceus appear to reflect the heterogeneity of both host and parasite populations, particularly with respect to variations in host susceptibility and to parasite searching activity. In adult P. georn tra, the parasitic phase is influenced by density-dependent factors with increasing intraspecific competition at high densities. The population processes controlling these parasites are compared to other types of host/parasite systems.