Targets of rapamycin (TORs) are conserved phosphatidylinositol kinase-related kinases that are involved in the coordination between nutritional or mitogenic signals and cell growth. Here we report the initial characterization of two Schizosaccharomyces pombe TOR homologs, tor1 + and tor2+ · tor2+ is an essential gene, whereas tor1+ is required only under starvation and other stress conditions. Specifically, Δtor1 cells fail to enter stationary phase or undergo sexual development and are sensitive to cold, osmotic stress, and oxidative stress. In complex with the prolyl isomerase FKBP12, the drug rapamycin binds a conserved domain in TORs, FRB, thus inhibiting some of the functions of TORs. Mutations at a conserved serine within the FRB domain of Saccharomyces cerevisiae TOR proteins led to rapamycin resistance but did not otherwise affect the functions of the proteins. The S. pombe tor1+ exhibits different features; substitution of the conserved serine residue, Ser1834, with arginine compromises its functions and has no effect on the inhibition that rapamycin exerts on sexual development in S. pombe.