Total mortality, mortality due to predation, and the contribution of parasite-induced mortality were estimated for yellow perchPerca flavescensinfected with the nematodeRaphidascaris acus. Total annual natural mortality of yellow perch in Dauphin Lake, Manitoba, was approximately 61%; predation by northern pikeEsox luciusaccounted for 19% and 45% of natural mortality for female and male yellow perch, respectively. Yellow perch harbored 18 species of parasites, of whichR. acuswas the most abundant; 95% of allR. acuslarvae inhabited the liver. The number of free and encapsulated larvae and nodules in the liver of yellow perch varied seasonally and with fish gender, but it was not correlated with water temperature or fish habitat. Mean intensity but not recruitment increased with age of yellow perch. Density ofR. acus(larvae per gram of liver) was highest in young, immature yellow perch and decreased with age in mature yellow perch. Yellow perch with high-density infections had lower condition factors, and their weight-at-age curves showed reduced slope (males and females) or a shift toward the ordinate (males). Reduced weight was most pronounced when densities of larvae were high (>50 larvae/g liver). Parasite-induced mortality accounted for much of the unexplained natural mortality of yellow perch. Many small yellow perch males with high-density infections apparently failed to mature; those that did mature had reduced weight. Females with high-density infections either died soon after the onset of vitellogenesis or matured later. Sex-related, parasite-induced mortality might explain the preponderance of females and the bimodal weight distribution that develop with age in natural populations of yellow perch. Furthermore, parasites (especially tissue-invasive types) may play a significant role in the regulation of yellow perch populations.