Summary.Observations from the jetty of the Biological Station, Bermuda, of the appearance ofAnchistioides antiguensis(Schmitt) at the surface over a short period in 1934 and eight months in 1935 are recorded, and show definite periodicity sometimes of sufficient intensity to merit the term swarming.The periodicity coincides with the two weeks of the lunar cycle when the moon wanes from the last quarter to the first quarter of the new cycle. It appears to consist of two maximum appearances separated by an interval of some days.Direct lunar influence, sex and the reproductive period, the surface‐temperature, wind and weather, and the state of the tide are shown to have no connexion with swarming.An explanation is suggested based on the feeding habits and depending on the periodic activity of the worms that form the food. The double maximum of the swarming period is shown to represent two stages of growt