A description of the development at 27�C. of Ceylonocotyle streptocoelium (Fischoeder 1901) Nasmark 1937, a paramphistome from the rumen and reticulum of sheep and cattle in Australia, is given in detail. Eggs hatched after 16 days' incubation and miracidia penetrated the intermediate host, a planorbid snail, Glyptanisus gilberti Dunker 1848, through the mantle cavity. The development of the larval stages of the parasite within the intermediate host was completed 34 days after infection, when cerariae were secreted by snails. Cysts of C. streptocoelium fed to sheep and cattle showed that flukes reached maturity in the rumen in 48 and 56 days respectively. The life cycle of the fluke, therefore, was completed in a minimum period of approximately 3.5 months under these conditions. It was found that temperature had a marked effect on the rate of development of larval stages within the molluscan intermediate host, the developmental period at 20�C. being twice as long as that at 27�C. The various stages occurring in the molluscan intermediate host are described in detail. The development of Paramphistomum ichikauiai Fukui 1922 is compared with that of C. streptocoelium. The rate of development was found to be more rapid than for C. streptocoelium under the same conditions. Eggs hatched after 12 days incubation and cercariae were secreted from the intermediate host, Segnitilia alphena Iredale 1943, 25 days after infection. Flukes were found to reach maturity in sheep 49-51 days after cysts had been fed. The eggs and intra-molluscan stages are similar in both species, but the cercariae may be readily distinguished by anatomical differences in their excretory systems. The life cycle in this species takes approximately 3 months to complete under these conditions. The molluscan intermediate hosts of the two species of amphistomes are described and notes on their bionomics are given.