Zearalenone, a mycotoxin secreted by Fusarium sp. and Gibberella zeae, shows a narrow range of antibacterial activity limited to some Gram-positive aerobic spore-forming bacteria. In Bacillus thuringiensis (Berliner), a highly sensitive species, this activity is characterized by a decrease of cellular division and induction of atypical cells. These effects resemble those obtained with two other mycotoxins which possess a lactone structure: aflatoxin B1 and patulin.