Growth and feeding rates in Stentor coeruleus fed on Tetrahymena are described in relation to prey concentrations ranging from 10:1-50:1 at 15°C and 20°C. Feeding rate was independent of temperature, but governed by prey concentration. Stentor decreased in size where less than 1,000 μJ were consumed in 24 hrs. A maximum level of growth was attained at 15°C, but not at 20°C within the range of prey concentration offered. Energy budgets, constructed using respiratory data published elsewhere (Laybourn, 1975), are presented. Assimilation efficiencies ranged from 74.4-76.4% at 20°C and 65.1-83.2% at 15°C. Gross growth efficiencies were between 70.9-73.6% at 20°C and 63.7-81.6% at 15°C. Net growth efficiencies were very high, 95.6-96.3% at 20°C and 97.5-97.9% at 15°C. The results are explained and discussed in relation to other Protozoa.