paramphistoines {parampbistomum sp.) were found in 86% of 160 moose rumens collected from northwestern ontario at all seasons of the year. all moose older than 2.4 years were infected. the number of flukes per moose ranged from 16 to 28,262 (median = 1,135). their distribution in moose was overdispersed and approximated a negative binomial. the intensity of infection did not vary with age with the exception of calves (0.5 - 1.4 years) which had fewer flukes than older moose. small, newly acquired flukes were first seen in the rumen of calves and older moose in october. few gravid flukes were found in moose over winter. the proportion of worms with eggs increased in march and april and 100% were gravid from may to july. thereafter, the proportion gravid declined, reaching 0.5% in november. similarly, few eggs were found in feces of wild moose during winter but the numbers began to rise in march and exceeded 100 eggs/g feces in july. by september, few eggs could be found. this annual fluctuation in the production of eggs is not related to seasonal transmission and life span of the flukes. experimentally infected moose which had no opportunity for reinfection began to pass eggs in spring and stopped in the fall for up to 3 consecutive years following a single infection. this adaptation, which enables the parasite in a northern host to recognize favourable seasons, may compensate in part for the inability of paramphistome eggs to survive freezing. three captive moose given 3,000 to 11,750 paramphistome metacercariae in september began to pass eggs the following april (185-225 days). another, not given metacercariae (9,000) until december, came patent in 169 days (may). one experimental animal given non-gravid flukes from a wild moose by rumenotomy in late february also began to pass eggs in april. no signs of disease due to rumen fluke infections were seen in experimental animals. a total of 7,910 aquatic snails representing 15 species were examined from 32 lakes and rivers thought to be used by moose. only helisoma trivolvis and h.campanulatum were shedding paramphistome cercariae (0.9% and 1.1% respectively). paramphistome metacercariae were found attached to the ventral side of floating vegetation in 4 of 12 lakes investigated. the black, pin-head-sized metacercariae were present on vegetation from late june until late august and september when aquatic plants began to die. attempts to infect h. trivolvis and h. campanulatum in the laboratory were unsuccessful. paramphistome eggs held at 11 degrees c did not develop. when the temperature was raised to 19 degrees c, eggs began to hatch after 30 days. since lakes in northwestern ontario do not reach 11 degrees c before mid-may it is most likely that metacercariae appearing on aquatic vegetation in late june originate from snails infected in previous summers. transmission to moose is probably greatest when they feed most intensively on aquatics from mid-june to mid-july.