Abstract5‐Chloro‐3‐methyl‐4‐nitroisothiazole (III) was prepared by nitration of 5‐chloro‐3‐methyliso‐thiazole. Compound III was found to exhibit significant antifungal activity invitroagainst a wide spectrum of fungi. The synthesis of 3‐methyl‐4‐nitro‐5‐nitroamino, 5‐carboxamido, 5‐N,N‐dimethylamino and 5‐β‐hydroxyethylaminoisothiazole are here reported. The synthesis of 3‐methyl‐4‐nitroso‐5‐ethylthioisothiazole (IX) is reportedviaan unusual reaction of 5‐bromo‐3‐methyl‐4‐nitroisothiazole (I) and sodium ethyl mercaptide. 5‐Bromo‐4‐nitroisothiazole was prepared by nitration of 5‐bromoisothiazole.