1. Observations on Fasciola hepatica in Limnaea truncatula under laboratory conditions showed that the cercariae did not emerge at temperatures below 9°C. Above 9°C. emergence proceeded indiscriminately up to the limit of experimental observation (26°C).2. Experience showed that emergence occurred in the light or in darkness while changes in the hydrogen-ion concentration of the water containing the snails (within the range 5.5 to 8.5) had no effect on emergence. There was some evidence that an increase of carbondioxide in the water slowed but did not inhibit emergence but depletion of oxygen had no apparent effect.3. In the laboratory, emergence could be induced by removing snails from dry habitats and immersing them in water or by removing snails from watery habitats and placing them in fresh water. We were unable to determine the exact physical.or chemical factors acting as a stimulus under such conditions.

1. This paper describes the life-history of Limnaea truncatula under laboratory conditions.2. Under controlled conditions sexual maturity may be reached in 28 days and egg laying continues for the greater part of the snail's life.3. Under good environmental conditions each snail may lay as many as 60 eggs in a single day and a total of more than 3,000 in a life approximately a year.4. Egg laying occurs during every month and at temperatures as low as 10°C. to 11°C. The incubation period of the egg is related to the atmospheric temperature.5. The longevity of the snail is likely to be related to its rate of growth and to environmental conditions including periods of drought.6. The snail remains fully active at temperatures as low at 1.5°C.Sustained temperatures much above 20°C. are unfavourable, temperaturesabove 25°C. proving markedly deleterious.