walleye (stizostedion vitreum vitreum ) and yellow perch (perea flavescens ) from lake of the woods were examined for parenteral proteocephalus ambloplitis from may to november, 1982 and 1983. one hundred percent of the age 1 and older walleye and 74% of the age 1 and older yellow perch harboured plerocercoids. in corresponding age classes, walleye were generally 10 times more heavily infected than yellow perch. mean intensity of live plerocercoids increased with age of walleye until age class 5 then declined significantly in older fish. intensity increased from a mean of 23 (in age class 0) to a maximum of 171 in age class 5 and was only 58 in the 7+ age class. mean intensity of plerocercoids increased continuously with the age of yellow perch from 2 (age class 0) to a maximum of 20 in the 5+ age class. all age classes of walleye (0 to 7+) became infected by preying on yellow perch, particularly the young-of-the-year (yoy). small yellow perch became infected by eating copepods but older perch obtained plerocercoids by cannibalism. the transmission of plerocercoids to walleye and yellow perch was greatest during late summer. young-of-the-year walleye and yoy yellow perch first harboured plerocercoids in early august. the liver was the first organ of walleye and yellow perch to be invaded by migrating plerocercoids. however, in walleye, the mesenteries ultimately contained the greatest proportion of p1erocercoids in age 1 and older fish. the liver remained a relatively important site for plerocercoids in all age classes of yellow perch. relatively few plerocercoids were found in the gonads of walleye or yellow perch. walleye fecundity was not correlated with plerocercoid intensity. ninespine sticklebacks ( fungitius pungitius ) and logperch (percina caprodes ) were found to harbour p. ambloplitis p1erocercoids . these are new host records. migrating plerocercoids caused the greatest pathological change in the liver of walleye and yellow perch. zones of compressed and necrotic hepatocytes were evident adjacent to live, unencapsulated plierocercoids . the mesenteries of walleye were often fibrosed in response to large numbers of invading plerocercoids. the gastro-intestinal tract, posterior gonads and associated mesenteries were often compacted with fibrous tissue. obstruction of the passage of gametes is possible. constriction of the oviduct may result from the fibrous reaction. also, encapsulated plerocercoids were found in the lumen of the oviduct creating a physical barrier. the wall of capsules encompassing plerocercoids in walleye was relatively thin (maximum of 90 microns) while in yellow perch, it was up to 290 microns thick.