Twelve new 3-(isobutyl)-5-substituted-tetrahydro-2H-1,3,5-thiadiaz- ine-2-thiones were synthesized by the reaction of isobutylamine with carbon disulfide and potassium hydroxide, followed by formaldehyde and appropriate alkyl, cycloalkyl aralkyl amines, amino acid, and INH. Their structures have been elucidated by spectral data and elemental analysis. The title compounds were tested for antimicrobial activity in vitro against gram-positive bacteria (Staphylococcus aureus,and Micrococcus leuteus), gram-negative bacteria (Serratia marcescens and Escherichia coli) and some fungi (Candida albicans, Scopulariopsis brevicalus, Geotrichum candidum, Macrophomina phaseolina, Fusarium oxysporum and Trichoderma harzianum) using agar cup diffusion method. The antimicrobial activity was found to be greatly affected by the bulkiness of the side chain and the presence of polar carboxylic group. Highest activity was obtained with compounds 4a and 4k (R= CH3, CH2-COOH).