Prevalence of neurologic disease attributed to Parelaphostrongylus tenuis was investigated in a population of wapii (Cervus elaphus canadensis) enclosed in a 2080 ha. preserve. Brains of 44 animals harvested in 1973, 1974 and 1975 were examined grossly and microscopically. Prevalence of infection increased each year from 26.6% of the sample in 1973 to 64.3% in 1975. These data suggest that wapiti living close to white-tailed deer (Odocoileus virginianus) may have a high prevalence of infection, but many individuals may not show signs. Accordingly, routine histologic examination of natural mortality cases without supporting observations of clinical signs would be inadequate to define cause of death. While the effects of the parasite in the population studied did not produce a high herd mortality or a sudden die-off situation, the apparent greater susceptibility of the younger age-classes to the parasite would have a detrimental effect on population recruitment and might limit the growth of wapiti herds living in proximity to white-tailed deer that harbor P. tenuis.