A lake population of the cyclopoid copepod Cyclops strenuus Fischer had a 1-year life cycle including a diapause from June to January. Dormant stages consisted of fertilized females, not previously described as a diapause stage, and a small fraction of cop V. The longevity of the sperm in a diapausing female was at least 6–7 months. Pre-fertilization is assumed to be an adaptive mechanism to assure fertilization when male abundance is low. This life cycle pattern also gives a shorter time-lag between diapause break and offspring development, and also the low total adult population exerts a lower predation pressure on their offspring. The flexibility of life cycle patterns of copepods is emphasized.