Individual flukes of the monogenetic trematode Gyrodactylus alexanderi, maintained at 15 °C on isolated fish (Gasterosteus aculeatus leiurus), gave birth to only two daughters, the first after an average of 1.6 days, and the second after 6.9 days. For at least 20 generations, the first daughter was produced every 1.6 days without cross-fertilization occurring. Maximum longevity at 15 °C was 28 days, but reached 71 days at 7 °C.The mean life expectancy of flukes away from fish was 1.8 days at 15 °C. This was shortened in the presence of young sticklebacks which preyed on the Gyrodactylus. One to 13% of flukes survived slow changes in salinity.Fish heavily infested with G. alexanderi died. Of 67 fish, 25–35 mm in length and carrying over 150 flukes. 54 died, compared with only 33 of 174 fish carrying fewer flukes. Deaths were not associated with infections of protozoa, fungi, or bacteria.