Pyrenocine A, a phytotoxin produced by Pyrenochaeta terrestris (Hansen) Gorenz, Walker and Larson, possesses general antibiotic activity against plants, fungi, and bacteria. Effective doses for 50% inhibition (ED50s) are 4 micrograms/mL for onion seedling elongation; 14, 20, 20, and 25 micrograms/mL for the germination of asexual spores of Fusarium oxysporum f. sp. cepae, Fusarium solani f. sp. pisi, Mucor hiemalis, and Rhizopus stolonifer, respectively. Pyrenocine A also inhibits the linear mycelial growth of both P. terrestris and F. oxysporum with ED50s calculated as 77 and 54 micrograms/mL, respectively. Gram-positive bacteria are more susceptible to pyrenocine A than Gram-negative bacteria. ED50s are estimated as 30, 45, and 200 micrograms/mL for the inhibition of growth of Bacillus subtilis, Staphylococcus aureus, and Escherichia coli, respectively, with Pseudomonas aeruginosa resistant to those concentrations tested. Pyrenocine A acts primarily as a biostatic rather than a biocidal agent with all organisms tested showing some degree of recovery when released from pyrenocine A. Pyrenocines B and C show little antibiotic activity in the bioassays performed.