The seasonal cycle of population dynamics and production of the dominant copepod Pseudocalanus elongatus in the southern North Sea was described using for the first time concurrent shipboard measurements of abundance, egg production and stage durations. Biological and physical parameters were measured from February 2004 to May 2005. Highest population abundance (592*10 3 n m-2) was recorded in June. Maximum egg production rate (EPR) was 9.1 eggs female-1 day-1 in April 2004. High EPR occurred at several stations where Chl a concentration was low, suggesting P. elongatus to be able to use alternative food resources such as microzooplankton and detritus or to maintain high production rates by using lipid reserves. Stage durations derived from molting experiments ranged from 1 day (CII) in February up to 9.2 days (CV) in April, with fastest development for most stages occurring in April. The increasing temperature in summer did not further shorten developmental times. Therefore, food might be an important determinant of development of P. elongatus in the North Sea. Estimated generation times in August based on shipboard measurements of stage durations were 2.3 times longer than predicted from laboratory investigations and point towards the importance of field measurements when calculating secondary production. © The Author 2008. Published by Oxford University Press. All rights reserved.