The worm development and egg laying pattern of Echinostoma hortense(Trematoda: Echinostomatidae) were studied in albino rats and the brief clinical course was observed in human volunteers. A total of 21 rats were infected with 20-69 metecercariae each and two humans were with 7 and 27 metacercariae, which were collected from loaches. For recovery of worms, the rats were sacrificed at irregular intervals from the 6th to 150th day after infection and the human volunteers were treated with praziquantel and purged with magnesium salt on the 26-27th day. The stools of the rats and humans were examined for the eggs. The results were as follows: The worm recovery rate from the rats was not affected by the increase of infection time but varied individually; 9.1-50.0% (31.1% in average). From humans, 14.3% and 37.0% (32.4% in average) of challenged were recovered. In the rats, it was revealed that the worms rapidly grew for the first 14 days to become 7.59 mm in average length and 1.17 mm in average width but the growth became much slower thereafer until the 150th day; 7.95 mm in length on the 21th day, 9.04 mm on the 28th day, 10.21 mm on the 49th day and 12.62 mm on the 150th day. During the early stage of infection, the growth of genital organs(male or female) was expressed as sigmoid curves whereas non-genital organs(such as suckers) was simply as straight lines. The prepatent period of this fluke was 10-12 days in the rats and 16-17 days in men. After the start of oviposition, the egg production by the worms remarkably increased, reached maximum on the 32-33th day, followed by decrease thereafter. The maximum value of E.P.G./worm was 390. The major subjective symptoms in human volunteers were abdominal pain and diarrhea during the early stage of infection. The results show that human is as susceptible as the rats to E. hortense infection and the amount of egg production in the rats is greatly affected by the age of worms.