1. The spawning season of Cumingia extends from the middle of June until the middle or end of September. The most active spawning usually includes the last week in June and ends about August 20. Each female spawns more than once and the production of gametes is practically continuous.2. The spawning is heaviest from full moon to new moon and least at first quarter thus showing a lunar periodicity.3. Cumingia is greatly affected by shock and rough treatment so that sexually mature individuals spawn promptly when brought to the laboratory, after being placed in a bowl of quiet sea water.4. There are apparently no chemical stimuli by which the sexes stimulate each other to sexual activity as is the case in Nereis limbata and the oyster. They spawn as readily when isolated as when in close proximity.5. It is shown that temperature is not the only factor which determines the duration of the spawning season and periods of spawning.