Data are presented which show that the microbiological degradation of phenol in industrial wastewater is affected by temperature in an unexpected manner. The rate of degradation is unaffected by temperature changes in the range from 24 to 10 °C but falls off rapidly at temperatures below 10 °C. In the interval from 10 to 2 °C the rate of degradation is a function of temperature and is proportional to the rate of growth of the bacteria studied. By contrast, the increase in rate of phenol degradation is much greater than the increase in growth rate as the temperature increases from 10 to 24 °C. Field and laboratory studies show that increased efficiency of phenol degradation in wastewater is due to increase in temperature rather than to shifts in microbial populations.