A specific deficiency in UDPG-linked trehalose-6-phosphate synthase in Saccharomyces has been associated with a single nuclear gene SST1 (M.S. Operti et al., Current Genetics, 5: 69-76, 1982). The sst1 phenotype was first observed in a pleiotropic fdp mutant which does not grow on mannose, sucrose, glucose or fructose, and in its partial spontaneous revertant (Q6R2) which acquired the capacity to grow on glucose but retained the mutant phenotype with respect to fructose. The sst1 mutants described in this paper are genetically derived from this partial revertant, a fact which suggested a link between sst1 and fdp mutations. All sst1 strains tested showed reduced growth on fructose in 24 h, which also characterizes the fdp mutation. Linkage studies revealed that the SST1 gene maps very close to the FDP gene. Furthermore, another mutant (cif) described as similar to fdp also lacked trehalose-6-phosphate synthase activity. Complementation tests between cif and fdp mutants indicated that the genes are allelic. These results suggest that SST1 and FDP are allelic and that the sst1 mutation might not be associated with the structural gene for trehalose-6-phosphate synthase but corresponds to one of the effects of the pleiotropic fdp mutation.