The variations in the maximal specific growth rate of the psychrotrophic bacterium Pseudomonas fluorescens MF0 with respect to temperature were studied between 0 and 30 degrees C (optimal for growth). The Arrhenius plot showed a drastic change in slope at the intermediate temperature of 17 degrees C. Over the cold domain from 0 to 17 degrees C, the temperature characteristic was twofold higher than over the suboptimal domain from 17 to 30 degrees C. The macromolecular composition of exponentially growing cells was invariant over the entire range from 0 to 30 degrees C. Variations of temperature and growth rate were independently investigated through chemostat experiments in order to characterize their respective effects on cell macromolecular composition and size. The effect of growth rate in this psychrotrophic strain is identical to that of all other bacteria assayed so far. In contrast, an original biphasic variation of total protein concentration was demonstrated in strain MF0 with respect to temperature, with a maximum at 17 to 20 degrees C. Indeed, increasing the temperature in the chemostat resulted in a biphasic decrease in the net protein production rate: a very slight decrease below 17 degrees C and a much larger decrease from 17 to 28 degrees C. These results could signify an increase in the cellular protein degradation rate with increasing temperature, especially above 17 degrees C.