The influence of temperature on yield, maintenance rate, growth rate, and conversion of calories to biomass was studied with Pseudomonas fluorescens grown in a chemostat. Maintenance and growth rate are influenced linearly with temperature. Both rates increased with increasing temperature and gave linear Arrhenius plots over a limited range. Cells harvested during the steady-state at each temperature were burned in a microcalorimeter. The number of kilocalories per gram (dry weight) of organism was not influenced significantly by the temperature during growth, indicating that the conversion of substrate calories into biomass is apparently regulated in the range of temperature studied.