New 2-monosubstituted and 2,2-disubstituted 2,3-dihydro-3,5-diphenyl-1,3,4-thiadiazoles (2) were synthesized by the reaction of N'-phenylthiobenzoylhydrazide with aliphatic, aromatic and heterocyclic aldehydes and ketones in the presence of trimethylsilyl chloride in benzene. The reaction proceeded smoothly to afford the corresponding 1,3,4-thiadiazoles (2a-2j) in excellent yields (84-96%) except for a few cases, such as the 5-methyl-2-furfural and 2-acetylthiophene systems. 2,3-Dihydro,2,2-dimethyl,3,5-diphenyl-1,3,4-thiazole (2f) showed curative activity against powdery mildew of wheat.