In the psychrophilic bacterium Vibrio sp. strain ANT-300, which has the ability to grow efficiently between 13 and −2 °C, with an optimum at 7 °C, cells in steady-state growth at 0 and 13 °C appeared to exhibit different patterns in the levels of certain individual proteins. With a shift in temperature, the steady-state level of individual proteins was achieved only after dramatic transient changes in the rates of synthesis of a small number of those proteins whose levels would be adjusted. Upon a shift up from 0 to 13 °C, the rates of synthesis of at least 25 proteins increased transiently, while increased rates of synthesis of 39 proteins were induced immediately upon a shift down from 13 to 0 °C. The proteins of which the levels would be adjusted were synthesized at differential rates, which varied conspicuously with respect to timing after the shifts in temperature. Such changes appear to be active regulatory responses to changes in temperature. Key words: psychrophile, Vibrio sp. strain ANT-300, heat-shock proteins, cold-shock proteins.