Seasonal changes in phytoplankton biomass and production, total zooplankton biomass, and biomass and potential production rates of the two dominant copepods, Acartia hudsonica (formerly called Acartia clausi) and Acartia tonsa are described for several stations in Narragansett Bay, R.I. Plankton in the bay behaved as a single population with simultaneous changes occurring at the upper bay (Station 5) and the lower bay (Station 1). Phytoplankton biomass was higher in the upper bay ( $$\bar x$$ =16.95 mg chl a·m−3) than in the lower bay ( $$\bar x$$ =6.37 mg chl a·m−3) and these 0269 0101 V differences in biomass were reflected in the phytoplankton production rates. The zooplankton, which was dominated by A. hudsonica in the spring and early summer and A. tonsa during summer and fall, showed no such consistent differences between the stations. Mean A. hudsonica biomass (St 1, $$\bar x$$ ;=82.7 mg dry wt·m−3; St 5, _ $$\bar x$$ ;=95.2 mg dry wt·m−3) exceeded that of A. tonsa (St 1, $$\bar x$$ ;=56.7 mg dry wt·m−3; St 5, $$\bar x$$ ;=60.0 mg dry wt·m−3). Potential production rates of the two Acartia 0269 0101 V spp. were strongly temperature dependent. Despite the higher biomass levels of A. hudsonica, low temperatures resulted in lower potential production rates ( $$\bar x$$ ; St 1=7.25 mg C·m−3 day−1; $$\bar x$$ ; St 5=10.77mg C·m−3 day−1) and biomass doubling times of up to 9.6 days. Potential production rates of A. tonsa at summer temperatures were high ( $$\bar x$$ ; St 1=19.0 mg C·m−3 day−1; $$\bar x$$ ; St 5=22.9 mg C·m−3 day−1) and biomass doubling times were generally less than one day.