The non-protein amino acid @b-aminobutyric acid (BABA) protects the plant against various pathogens. Until now, this protection was described to involve the priming of pathogen-specific defence responses but a direct fungicidal effect was excluded. The present study shows that BABA inhibited mycelial growth and germination of the necrotrophic fungus Botrytis cinerea and affected Saccharomyces cerevisiae growth in a concentration-dependent manner. Variations of the fungicidal action at acidic pH and between different strains were observed.