The pigmy marmoset (Cebuella pygmea) smallest of the monkeys, showed a limited thermoregulatory range between 20 and 30[degree] C. Deviations of -3 and of +3[degree] C from the neutral diurnal level of 37.9[degree] C were observed at 7[degree] C and 41[degree] C ambient respectively. The large daily cycle in body temperature ( [DELTA] = 3.7[degree] C) closely followed the daily light cycle and showed a very low nocturnal minimum of 34.5[degree] C. The BMR was 10-20% below that expected for a mammal of this size (0.76- 1.01 cm3 O2/g-hr.), and was maintained over an unusually wide range (30-40[degree] C). The metabolic increment with temperature (thermal conductance) was within the average range for temperate small mammals (0.115 to 0.123 cm3 O2/g-hr.[degree]C).