Abstract Life cycles of the large suspension-feeding copepods, Eucalanus bungii and Neocalanus cristatus were investigated by seasonal sampling in the western subarctic Pacific. Eucalanus bungii has a diapause from August to March at copepodite stages between copepodite 3 (C3) and C6 female. We propose that individuals with early birth dates are young of overwintering C5 and C6-females that develop to C4 in their birth year, while individuals with late birth dates are young of overwintering C4 that develop to C3 in their birth year. Thus, a majority of the population has annual generations alternating with biennial generations. Neocalanus cristatus showed life history almost identical to the population in the Alaskan gyre. Timing of the life cycle in N. cristatus is very close to that in the eastern subarctic gyre, but that of E. bungii is 2 months earlier than in the eastern subarctic. This difference is probably caused by the timing difference in the maximum primary production in the two areas and the plasticity of the life cycle strategy in E. bungii.