The uranium metal temperature coefficient of reactivity has been measured in ZEEP. A uranium sample was oscillated in the reactor and the resulting modulation of reactor power was measured as a function of the sample temperature. The temperature coefficient of uniformly heated uranium rods, 3.25 cm. in diameter, immersed in a constant temperature moderator (moderator-to-uranium volume ratio 22) is deduced from this experiment. Over the range +30°C to +230°C the coefficient is dk/dT = − (1.25 ± O.09) × 10−5 per °C. Over the range +10°C to −140°C the coefficient is dk/df = −(1.58 ± 0.18) × 10−5 per °C.