Eggs of Polystoma integerrimum incubated in alternating periods of light and darkness at a constant temperature hatch in the light. 2. Eggs incubated and hatched in continuous light (10 degrees or 18 degrees C) hatch without a rhythm. Eggs incubated and hatched in continuous darkness at 18 degrees C hatch without any circadian rhythm but fail to hatch at 10 degees C until 1 month later than expected. 3. At both 10 degrees and 18 degrees C the hatching rhythm persists in conditions of continuous light if, during incubation, the eggs were exposed to alternating light and darkness. At 18 degrees C the rhythm appears to persist in total darkness following incubation in alternating light and darkness. At 10 degrees C hatching is blocked. 4. At 10 degrees C if eggs are incubated in alternating periods of light and darkness and then, when hatching begins, these periods are reversed, the hatching rhythm becomes synchronous with the new lighting regime. This may be due to the effect of low temperature. 5. Hatching during the day may have survival value of P. integerrimum since the host tadpoles congregate at this time around the edges of the lakes, the region where the parasite eggs are laid.
