elaphostrongylus rangiferi is an introduced parasite in caribou (rangifer tararuhts caribou) of newfoundland and has caused at least two epizootics of cerebrospinal elaphostrongylosis (cse), a debilitating, neurologic disease. to understand the conditions necessary for such outbreaks, two hypotheses were investigated. first, i examined whether parasite abundance was primarily determined by herd density or climatic conditions. the abundance of e. rangiferi was represented by counts of first-stage larvae in feces of calves and yearlings collected in february from nine distinct caribou herds in newfoundland. seven of the nine herds had concomitant infections of e rangiferi and another protostrongylid nematode, parelaphostrongylus andersoni. the cape shore and bay de verde caribou had only p. andersoni. abundance of e. rangiferi was highest among young animals (calves and yearlings) in the avalon (x =632 ± 14) and st. anthony (x =526 ± 145) herds during february. reports of cse were most frequent in these two herds. abundance was correlated positively with mean annual minimum temperatures (r,=0.829, df=6, p=0.04), and the number of days per year above 0 degress c (r,=0.812, df=6, p=0.05) and negatively with mean summer temperatures (r,= -0.830, df=6, p=0.04). abundance was not correlated with herd density. it was also hypothesized that young animals develop an immunity to e. rangiferi that prevents re-infection later in life. this was examined by pressing the brains of known-age caribou to detect recently acquired e. rangiferi. worms were found on the brains of young caribou but not in animals older than two years, except for those of the avalon herd. the continued infection of older animals in the avalon herd may be due to lower immuno-competence of animals in a herd only recently infected with e. rangiferi. this study also examined the usefulness of abomasal parasite counts (apc) (trichostrongyloidea) in predicting herd density. three species of trichostrongylid nematodes were present: ostertagia gruhneri, trichostrongylus axei and haemonchus contortus; o. gruhneri predominated. there was no significant correlation between mean apc and herd density (r,= -0.40, df=4, p=0.60). however, further analysis indicated that worm burden was influenced by climate.