Fourteen-day-old Atlantic silve sides were exposed over a period of four weeks to four different constant or cyclic temperature regimes. The four regimes used were: 'ambient'(A) - ambient temperature (17.8 to 21.9C) throughout the experiment; 'heated'(H) - 8 + or -1C above ambient throughout the experiment; 'ambient 1, heated 2' (H1,A2) - ambient during weeks 1 and 3, 8 + or -1C above ambient during weeks 2 and 4; and 'heated 1, ambient 2' (H1,A2) - 8 + or -1C above ambient during weeks 1 and 3, ambient during weeks 2 and 4. Measurements of growth (wet weight) made during this period suggest that Atlantic silversides hatching in the warmer waters of upper marsh and tidal rivers would grow at a greater rate, other things being equal, than fish hatching in colder Narragansett Bay waters.