ABSTRACT: In situ field observations of the suspension-feeding North Atlantic sea cucumber Cucumaria frondosa from the Bay of Fundy, Canada, conducted over a 3 yr period, revealed a seasonal feeding rhythm. Sea cucumbersextended their tentacles and began feeding in March/April and ceased feeding in September/October. There were significant relationships between this feeding activity, daylength and the quality of the seston, but not its concentration. Together daylengthand chloropigment concentration explained 49% of the variability in the percentage of feeding sea cucumbers. Feeding activity was not significantly correlated with the distinct temperature cycle observed in the study area. Feeding activity was influencedby the state of the tide, water temperature and by daylight during some periods of the feeding season. However, for most of the year these factors appeared not to be important. Sea cucumbers increased their rate of tentacle insertion into the mouth as thequality of the seston increased. The rate of tentacle insertions increased with increasing seston chloropigment concentration but decreased with increasing current speed. These 2 environmental variables explained about 28% of the variability in tentacleinsertion rates. Seston quality is likely the major environmental variable influencing seasonal feeding behaviour and feeding rate in this species.