The only nutrient element other than nitrogen and phosphorus which limited microbial activity in a sandy soil was sulfur. The addition of extremely small quantities of MgSO 4 , ranging from 0.32 to 1.6 mg sulfur/100 g soil, markedly increased the rate of oxidation of glucose. The optimum concentration of sulfur was correlated with the levels of glucose added, and a C/S ratio of 900 or less was required for maximum respiration. A number of compounds containing sulfur at different oxidation stages and in various structural configurations readily satisfied the sulfur requirement, indicating that the response was to sulfur as a nutrient and not to sulfate as an electron acceptor. Thiourea and elemental sulfur were utilized only slightly. The differential utilization of the various sulfur-containing compounds and the implications of sulfur as a limiting factor of microbial activities in soil were discussed.