The formation of volatile compounds during the Bacillus subtilis fermentation of cooked, roasted soya bean cotyledons was investigated. The materials examined were : dry roasted cotyledons ; autoclaved, roasted cotyledons ; and autoclaved, roasted cotyledons fermented for 18 h and 36 h at 35Β‘C. Growth of B subtilis led to the formation of many volatile compounds. The volatile compounds formed in the largest amounts during fermentation were 3hydroxy-2-butanone (acetoin), 2,5-dimethylpyrazine and trimethylpyrazine. Compounds present at concentrations exceeding their odour threshold values included nonanal, decanal, 1-octen-3-ol, butanedione, 3-hydroxy-2-butanone, 3octanone, 2,5-dimethylpyrazine, 3,6-dimethyl-2-ethylpyrazine, 2-pentylfuran, dimethyl sulΓde, benzaldehyde and 2-methoxyphenol. Compounds found in 18 h fermented cotyledons which were absent, or present in much lower concentrations, in roasted or autoclaved cotyledons included several aliphatic ketones, acetic acid, two aliphatic esters, several pyrazines, 2-pentylfuran, dimethyl sulphide, 2-methoxyphenol and trimethyloxazole. The total mass of volatile compounds present after 36 h incubation was less than half that present after 18 h. This was mainly due to the disappearance of 3-hydroxy-2-butanone, 2,5dimethylpyrazine and 2,6-dimethylpyrazine. The biogenesis of the volatile compounds is discussed.