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We present evidence to show that the timing of the seasonal appearance and disappearance of Acartia hudsonica, the dominant winter-spring copepod in Narragansett Bay, a north temperate estuary, is controlled by temperature and its effect on reproductive rate and formation and hatching of resting eggs. Maximum reproductive rates and growth efficiency for this species were observed at temperatures below those typical of the mid-summer season when it is replaced by Acartia tonsa, the summer dominant. Furthermore, during early summer A. hudsonica produces eggs that do not hatch at the temperatures at which they are produced (> 16 °C) and that will hatch only after exposure to low temperature. These eggs were also present in the sediments of Narragansett Bay and are probably true diapause eggs. It also appears that competition for food is not an important factor controlling seasonal succession of the species. Populations of both Acartia species were observed in 13 m3 enclosed ecosystems which differed from Narragansett Bay in that food supply was increased by addition of nutrients. Timing of the seasonal transition between A. hudsonica and A. tonsa was not altered in any consistent way by these changes, although population size increased in some years.