Although cyclopoids of the genusOithonaare considered the most abundant copepods in the marine environment, there is still very little information about what sustains their population and almost constant reproduction rate throughout the year. Feeding and egg production rate (EPR) ofO. similiswere measured at coastal and oceanic stations during 3 cruises in the North Atlantic between April and November 2002.O. similisingested ciliates preferentially to other components of the nano- and microplankton (herein nano-microplankton), which only became a more important component of the copepod diet when the abundance of the former decreased to low concentrations. EPR did not show significant seasonal differences, with 2.13 ± 0.67 eggs female–1d–1in spring, 1.61 ± 0.32 eggs female–1d–1in summer and 1.60 ± 0.15 eggs female–1d–1in winter. The ingestion rates measured at many oceanic stations and in winter were often too low to sustain EPR. Egg production efficiency (GGE) >100% indicated that sustained EPR might have relied, particularly in winter, on alternative food sources such as a more carnivorous diet and/or on the faecal pellets of euphausids.