A number of 5-nitrothiazoles with various substituents in the 2-position were tested for inhibition of Clostridium botulinum in a culture medium. Thiazole itself or 2-bromo- or 2-methylthiazole at 30 mug/ml did not inhibit the organism. An amino group in the 2-position of thiazole inhibited at 10 mug/ml. Substitution of a nitro group in the 5-position of 2-aminothiazole increased the inhibitory level to 0.12 mug/ml; acetyl-, propionyl-, or butyroyl-2-amino-5-nitrothiazole inhibited at 0.04 mug/ml. Benzoyl-2-amino-5-nitrothiazole inhibited at 0.16 mug/ml; this increased to 0.01 mug/ml when the benzoyl group carried a nitro group in the m- or p-position; a nitro group in the o-position, on the other hand, inhibited at 0.04 mug/ml. Unsaturated aliphatic acyls decreased inhibition. The greatest activity was exhibited by 2-nonanoyl- and 2-lauroylamides, with minimum inhibitory concentrations of 0.005 and 0.0025 mug/ml, respectively.