Discocotyle sagittata displays a clearly defined nocturnal egg-hatching rhythm. When eggs of D. sagittata were incubated in alternating 12 h periods of light and darkness at 13 °C, the majority of larvae hatched within the first 2 h of darkness. Larvae were rarely recovered after 4 h of darkness, and none hatched in the light. Reversal of the light/dark cycle resulted in immediate reversal of the hatching rhythm, so that larvae emerged at the beginning of the new dark period. This suggests that hatching is a direct response to exogenous cues and not endogenously driven. In continuous darkness hatching was arrhythmic; hatching occurred in continuous light although total numbers of larvae emerging were suppressed. Field experiments confirmed that eggs exposed to the natural day/night cycle throughout development also demonstrated the precise hatching rhythm seen under controlled conditions. Mechanical disturbance played no part as a hatching stimulus. However, larvae hatched in the presence of both host mucus and gill tissue. D. sagittata does not show an egg-laying rhythm. The hatching responses have adaptive advantage related to the host's activity pattern. Trout are generally visual predators, inactive in darkness, providing a stationary target for emerging larvae responding either to darkness or to host hatching factors.