Diel periodicity was discovered in 4 life cycle events for the swarming cyclopoid copepod Dtoithona oculata: the molt to adult stage, mating, egg sac production, and egg hatching. The timing of these events was associated with swarming in which dioithonans form swarms at dawn and disperse at dusk. A laboratory time course experiment revealed that molting of copepodid stage five (CV) to adult (CVI) occurred between midnight and dawn (0600 h). In videotaped experiments of laboratory swarms, mating encounters were greatest at dawn and therefore would occur as soon as CV molted to adults and were present in swarms. In experiments with field collected swarms, 80% of egg sacs were produced by females between midnight and 0820 h the next morning, and 80% of the eggs hatched by the following night between midnight and 0500 h. Egg cycle duration (clutch duration plus mter-clutch interval) was 48 h, and the mean fecundity was 0.56 pairs of egg sacs d ' or 7.6 eggs d \ We suggest that predation is the main selective force for diel cycles of D, oculata, but that diel cycles in reproduction benefit the survival of nauplii, whereas diel cycles of molting, mating, and swarming benefit the reproduction and survival of adults.