It is proposed that the timing of overwintering diapause in multivoltine, marine copepods is optimised as in insects and limnic copepods. Theoretical models state that resting should begin a period equivalent to 1 generation before the environment becomes unfavourable. Computer simulations and field studies have shown this to apply to insects and limnic copepods. The present paper tests and discusses the model predictions in relation to seasonal environ~nental changes and life history events for populations of Pseudocalanus acuspes (Giesbrecht, 1881) and Acartia longiremis Lilljeborg in North Norwegian fjords. These copepods switch from developing populations, with normally developing gonads, to resting populations, with immature gonads, within a few weeks in early fall. Phytoplankton abundance and primary production decline later in the fall.