The annual variation in abundance of all developmental stages of the harpacticoid copepod Euterpina acutifrons (Dana) was studied in plankton samples collected fortnightly from May 1971 to May 1972 in the So Sebastio Channel, Brazil. The species occurred throughout the year, becoming scarce during the summer. Three distinct peaks of abundance were evident (July, December and April), the most pronounced being in July (winter). Generation time in the field (26–27C) was calculated as approximately two weeks. Small males were always more abundant that large males but densities peaked during the coldest months when the highest populational breeding peak occurred. Males usually outnumbered females, and young members (nauplii and copepodites) outnumbered adult forms. Results are compared with tropical, northern hemisphere and New Zealand data.