Summary: The germination of sporangiospores of Rhizopus arrhizus was investigated, and differing requirements found for the initiation of germination, spore swelling and germ-tube emergence. The initiation of germination, as indicated by the commencement of swelling and by the spores becoming permeable to methylene blue, requires the presence of glucose or fructose. Maximal spore swelling requires in addition the presence of a nitrogen source, PO4 3- and K+ or Na+. If these requirements are satisfied, the increase in spore diameter with time is approximately linear for 8 hr, implying the maintenance of a constant rate of water uptake per unit area of spore surface for this period. If germinating spores are transferred to a medium lacking glucose, swelling soon ceases. The rate of swelling is identical at widely differing osmotic pressures. It is suggested that water uptake by the germinating spore is an active process, requiring energy. Germ-tube emergence from some spores can be obtained with glucose alone, but for germ-tube production from all spores other nutrients must also be supplied. If glucose is present, spores can take up sufficient nutrients in 1–2 hr to permit complete germination in the absence of exogenous nutrients several hours later. Depending on conditions, germ-tube production can occur after either slight or massive spore swelling. The effect of anaerobic conditions on germination was also examined and found to permit only partial spore swelling and greatly diminished germ-tube production.