The prevalence and intensity of Parelaphostrongylus tenuis was determined by examining the head and a fecal sample from each of 379 white-tailed deer (Odocoileus virginianus) of known age that had been killed by vehicles in northeastern Minnesota (USA), November 1991 to May 1993. Small numbers of adult worms (mean +/- SD, 3.2 +/- 2.2; maximum, 13) were found in the cranium of 311 (82%); but over a third (118 of 311) of the infected deer were not passing larvae in their feces. Most occult infections were sterile because only one sex of the parasite was present. Adult P. tenuis were not found in the vertebral canal of deer. Prevalence of adult worms and larvae was lower in fawns (68% and 35%, respectively) than in older age classes of deer (89% and 63%, respectively). Forty-three of 45 deer between 7 and 15 yr old were infected. Mean (+/- SD) intensity of adult worms was lower in fawns (2.7 +/- 1.8) and yearlings (3.0 +/- 2.1) than in deer 7 to 15 yr (4.1 +/- 2.5). Conversely, the mean (+/- SD) number of larvae in feces was higher in fawns (103 +/- 119 larvae/g) than in adults 2 to 6 yr old (36.2 +/- 46 larvae/g) and 7 to 15 yr old (35.6 +/- 60 larvae/g). Mean (+/- SD) fecundity of female worms was greatest in fawns (51.6 +/- 64.8 larvae/g of feces/female worm). Deer of all ages passed more lavae in the spring. Deer from an area where year-round density was 30 deer/km2 had a mean (+/- SD) of 3.5 (+/- 1.8) adult worms; deer from the study area, with a summer density 2 deer/km2, had 3.2 (+/- 2.2) worms; however, deer at the greater density passed a greater mean number of larvae (93.8 and 57.1 larvae/g, respectively). Based on our results we propose that P. tenuis is a long-lived parasite and that most deer become infected in their first or second summer of life, and acquire few additional worms thereafter.