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The life cycle of Paramphistomum microbothrium Fischoeder, 1901, in experimentally infested sheep, goats and cattle was compared and discussed. It is concluded, that of these domestic ruminants, cattle are the normal definitive hosts of P. microbothrium. The worms in cattle grow larger, migrate more rapidly, lay more eggs, live longer and the percentage take after migration is higher than that in either sheep or goats. Massive infestation of sheep and cattle resulted in retarded growth and migration of the worms. Metacercariae exposed to X-rays were dosed to sheep and the effects on the life cycle studied. The pathological anatomy, clinical pathology and symptoms of acute paramphistomiasis are described in detail and their inter-relationship is discussed. Adult cattle and sheep were successfully immunized by repeated dosage of small numbers of metacercariae (500 to I,500). Adult cattle were readily immunized either with single doses of 40,000 to 176,000 un-irradiated metacercariae or with single or divided doses of 40,000 irradiated metacercariae. Although adult sheep could be immunized, this was only completely successful with a single or divided dose of 40,000 metacercariae. Immunization of adult goats was successful with a single or divided dose of 40,000 metacercariae. The effects of immunity on paramphistomes are a marked reduction in numbers, retarded growth and rate of migration. The intradermal allergic test, a modification of the complement fixation test and the formation of precipitates around live worms incubated in sera from infested hosts were investigated.