twenty-one species of helminths were recovered from wild and captive caribou (rangifer tarandus). woodland caribou of the slate islands, barren-ground caribou of the beverly herd and captive woodland caribou were parasitized by 7, 7 and 16 species of gastro-intestinal helminths respectively. the predominant nematode recovered from all wild caribou was ostertagia gruhneri. the lung worm, dictyocaulus viviparus, was recovered from both wild and captive woodland caribou. the large number of species recovered from captive caribou suggested that cross transmission of parasites between captive caribou and a variety of other ungulates was common. ostertagia gruhneri appears to be a polymorphic species with o. arctica representing its minor form. the minor form never comprises more than 10% of the total number of male ostertagia, has stout, heeled spicules and sjoberg’s organ. the wide spread occurrence of polymorphism among the ostertagiinae suggests that a re-evaluation of the characters used to define genera and species is required. the average number of adult abomasal nematodes (3247) recovered from caribou of the slate islands was higher than reported for other wild cervids in north america. the number of abomasal nematodes present in animals was related to herd density. adult worms were more numerous during the spring and fall (x=4370) than in the winter (x=1280). inhibited fourth-stage o. gruhneri were found in wild caribou from the slate islands and comprised up to 85% of the total worm burden during the winter. no apparent disease caused by any helminth was observed in wild caribou. captive caribou subjected to various forms of stress developed heavy infections of abomasal nematodes (>20000) which appear to have been partially responsible for the death of two animals. seasonal fluctuations in the number of nematode eggs passed in the feces of wild and captive caribou were evident. fecal egg counts from slate, pic and otter islands and captive adult caribou were similar during the spring and summer. peak egg counts were observed in wild and captive animals during the fall. the low number of ostertagia sp. eggs passed in the feces of caribou during the winter months is due to the presence of fewer, less fecund worms. the free-living stages of nematodirella spp. were more resistant to freezing and dessication than those of ostertagia spp.. eggs of nematodirella frozen for 11 months or more hatched, while those of ostertagia would not hatch after freezing for 1 day. infective larvae of nematodirella longissimespiculata also appeared to withstand freezing and dessication better than those of ostertagia spp. caribou appear to develop a well marked immunity to nematodes of the genera nematodirella, nematodirus and dictyocaulus. calves are usually the only animals infected. immunity to o. gruhneri was also evident. worms recovered from caribou calves of the slate islands were longer and more fecund than those recovered from adults at the same location. the relationship between host age, herd density and worm morphology suggested that woodland caribou of pic island are not as heavily parasitized as caribou from the slate islands. stresses of the rut, injury or relocation appear to compromise acquired immunity to gastro-intestinal nematodes. anthelmintic treatment of captive caribou with ivomec eliminated patent infections of ostertagia spp., trichuris sp., capillaria sp. and oesophagostomum venulosum. ivomec was not effective against nematodirus sp.. treatment appeared to be effective against inhibited larvae of abomasal nematodes. ivomec may be useful as a prophylactic measure when transferring wild caribou. wild moose {alces alces) from northwestern ontario were parasitized by two species of gastro-intestinal helminths. ninety-six percent of moose examined were infected with nematodirella alcidis. the average number of worms recovered from wild moose was 111, considerably lower than recorded in caribou. the majority of worms recovered in most infections were immature. this appears to be the first report of inhibited fourth-stage n. alcidis. abomasal nematodes were not recovered from wild moose of northwestern ontario.