the morphology of elaphostrongyline nematodes parasitizing north american cervids was studied using light and scanning electron microscopy in order to improve previous descriptions that were based exclusively on light microscopical studies. structures including the spicules, gubernaculum, bursa, cephalic region, sensory structures, and female tails of parelaphostrongylus tenuis, p. odocoilei, p. andersoni, and elaphostrongylus cervi rangiferi are described. the morphology of e. c. rangiferi as described in this study is similar to previous light microscopical descriptions. the spicules of all three parelaphostrongylus spp. were alike but differed from those of e. c. rangiferi by having a dorsal branch of the spicule shaft in the distal region that extends to the tip. a membrane spans the area between the branch and the shaft. the corpus of the gubernaculum of p. odocoilei is not split distally as has been described by previous authors, but instead has a longitudinal groove in the distal region. ventral protuberances associated with the cloaca are present in parelaphostrongylus spp. but absent in e. c. rangiferi. sensory papillae and pores associated with the cloaca and the bursal rays are described. in dorsal view, the base of the dorsal ray has a prominent edge of thickened cuticle in p. odocoilei and p. andersoni, whereas in e. c. rangiferi and p. tenuis it is smooth. the female tails of parelaphostrongylus spp. have a papilla-like projection at their tips. the phylogeny of the elaphostrongylinae was reconstructed using cladistic techniques. morphological characters described in this study provide strong evidence for the monophyly of parelaphostrongylus. the genus is made up of two clades, one containing p. odocoilei and p. andersoni, the other being p. tenuis. the biogeography of the cervidae of north america in relation to the phylogeny of the elaphostrongylinae is discussed. the genus parelaphostrongylus probably coevolved with the genus odocoileus in the nearctic, while elaphostrongylus spp. most likely had a palearctic origin.