A series of organotin 4-methyl-1,2,3-thiadiazole-5-carboxylates and benzo[1,2,3]thiadiazole-7-carboxylates have been synthesized and characterized by NMR ((1)H, (13)C, and (119)Sn), IR, and elemental analyses. The structure of the dimeric complex {[(BTHCO(2))SnEt(2)](2)O}(2) (BTH represents benzo[1,2,3]thiadiazol-7-yl) has been further confirmed by X-ray diffraction crystallography. Assessment for fungicidal activity indicates that all of the newly synthesized compounds exhibit good growth inhibition against Alternaria solani , Cercospora arachidicola , Gibberella zeae , Physalospora piricola , and Botrytis cinerea . High growth inhibition percentage at 50 microg/mL was obtained in vitro in the case of triorganotin 4-methyl-1,2,3-thiadiazole-5-carboxylates and benzo[1,2,3]thiadiazole-7-carboxylates. The corresponding EC(50) values of these triorganotin carboxylates have been detected, and values of EC(50) as low as 0.12 microg/mL against P. piricola and 0.16 microg/mL against G. zeae, respectively, were observed for triethyltin benzo[1,2,3]thiadiazole-7-carboxylate.