The life cycle of a monogenean, Diclidophora sp., was studied with special attention to the time required for developmental stages to occur. Eggs are produced by adult worms at the rate of one every 13.5 minutes and require 32 days to hatch when incubated at 12.5��C and 30.9��� salinity. Rate of development and hatching success are strongly dependent on incubation temperature and salinity. Growth and development of the larval stages are similar to other known species of the family Diclidophoridae. The presence in the oncomiracidium of a precocious set of attachment clamps and the premature loss of larval hooks distinguish it from related species. Oncomiracidia survive approximately 36 hours if no host fish is reached. Larvae attach to the inner lateral borders of primary lamellae of the host fish gill. A second set of clamps is added before the 36th day, the third set soon after the 44th day, and the last pair by the 58th day. Sexual maturity is reached by the 153rd day after hatching. Experimental infections were maintained on redtail surfperch for 203 days. Naturally infected redtail surfperch, silver surfperch and walleye surfperch were collected. Rates of infection with Diclidophora were 38.5%, 50.0%, and 8.0%, respectively. Multiple infections were common. Female redtail surfperch were less often infected than were males. Age-dependent immunity was not observed. Both sexes demonstrated a positive correlation between the rate of infection and age of fish. White seaperch, shiner perch, pile perch and striped seaperch were not naturally infected. Redtail, silver, and walleye surfperch and white seaperch were experimentally infected and normal development of worms on these fish was observed. Oncomiracidia of Diclidophora sp. were exposed to various concentrations of blood serum and body mucus from redtail surfperch and striped seaperch, and their responses were observed. Oncomiracidia were more strongly affected by materials from the striped perch than from redtail surfperch. In both species mucus affected the oncomiracidia more strongly than blood serum. Adult worms were more severely injured by exposure to high concentrations of striped perch mucus or serum than by exposure to similar concentrations of redtail mucus or serum. Ring tests, Ouchterlony gel diffusion and passive hemagglutination tests were used to determine whether antibodies produced by striped perch were responsible for preventing attachment and growth of Diclidophora sp. Ring tests were negative, gel diffusion tests demonstrated very weak antibodies in both redtail surfperch and striped seaperch mucus. Passive hemagglutination experiments indicated weak antibodies to Diclidophora sp. in mucus and serum of redtail surfperch and in serum of striped seaperch. Mucus from the later species demonstrated recognizably stronger antibody titers. Mucus of both fish species contain weak concentrations of non-specific antibodies. / Graduation date: 1973