In North America, adults of meningeal worm Parelaphostrongylus tenuis (Dougherty, 1945) and muscleworm Parelaphostrongylus andersoni Prestwood, 1972 occur in white-tailed deer (Odocoileus virginianus) of eastern regions (Anderson and Prestwood, 1981. In Diseases and parasites of white-tailed deer, W. R. Davidson, F. A. Hayes, V. F. Nettles, and F. E. Kellogg (eds.). Misc. Pub. No. 7, Tall Timbers Res. Stat., Tallahassee, Florida, pp. 266-317) and adults of P. andersoni and muscleworm P. odocoilei (Hobmaier and Hobmaier, 1934) occur in white-tailed and mule and black-tailed deer (0. hemionus subspp.), respectively, of the West (Pybus and Samuel, 1981, J. Wildl. Mgmt 45: 537-542; Platt, 1978. Ph.D. Thesis, Univ. of Alberta, Edmonton, Alberta, 233 p.). Parelaphostrongylus tenuis and P. andersoni are found concurrently in whitetailed deer of the southeastern United States (Prestwood et al., 1974, J. Wildl. Dis. 10: 404409) and, based on experimental work (Pybus, 1983. Ph.D. Thesis, Univ. of Alberta, Edmonton, Alberta, 185 p.), the potential exists for concurrent infections of P. andersoni and P. odocoilei in mule deer. First-stage larvae (L1s) of these nematodes are passed in the feces of the definitive host and, later, develop to the third (=infective)-stage (L3) in terrestrial gastropods. Clearly, the ability to distinguish these larval stages would be advantageous for parsitologists and wildlife biologists. Unfortunately, first-stage larvae of all 3 species are similar (Anderson, 1963, Can. J. Zool. 41: 775-991; Prestwood, 1972, J. Parasit. 58: 897902; Platt, loc. cit.). Prestwood (loc. cit.) distinguished L3s of P. andersoni from those of P. odocoilei by size and P. andersoni from P. tenuis by a slight swelling on the dorsal surface of the tail of P. andersoni. In the present study, we compared the L3s of all 3 species using light microscopy and found that the shape of the tails was different. Laboratory-raised Triodopsis multilineata (Say) (Pulmonata: Stylommatophora) were used as intermediate hosts. Snails were infected by placing them in a dish containing L,s on moistened filter paper. The number of L1s exposed to each snail ranged from 10 to 1,000. Larvae of P. tenuis were recovered from frozen feces of whitetailed deer collected in the Rachelwood Wildlife