Rheb GTPases represent a unique family of the Ras superfamily of G-proteins. Studies on Rheb in Schizosaccharomyces pombe and Drosophila have shown that this small GTPase is essential and is involved in cell growth and cell cycle progression. The Drosophila studies also raised the possibility that Rheb is involved in the TOR/ S6K signaling pathway. In this paper, we first report identification of dominant negative mutants of S. pombe Rheb (SpRheb). Screens of a randomly mutagenized SpRheb library yielded a mutant, SpRhebD60V, whose expression in S. pombe results in growth inhibition, G1 arrest, and induction of fnx1+, a gene whose expression is induced by the disruption of Rheb. Alteration of the Asp-60 residue to all possible amino acids by site-directed mutagenesis led to the identification of two particularly strong dominant negative mutants, D60I and D60K. Characterization of these dominant negative mutant proteins revealed that D60V and D60I exhibit preferential binding of GDP, while D60K lost the ability to bind both GTP and GDP. A possible use of the dominant negative mutants in the study of mammalian Rheb was explored by introducing dominant negative mutations into human Rheb. We show that transient expression of the wild type Rhebl or Rheb2 causes activation of p70S6K, while expression of Rheb1D60K mutant results in inhibition of basal level activity of p70S6K. In addition, Rheb1D60K and Rheb1D60V mutants blocked nutrient- or serum-induced activation of p70S6K. This provides critical evidence that Rheb plays a role in the mTOR/S6K pathway in mammalian cells.