In a sensitive strain of Bacillus thuringiensis (Berliner), aflatoxin B1 inhibited growth with a dose above 5.0 microgram/ml. With subinhibitory levels (0.5 - 5.0 microgram/ml) physiological damage (decreased growth rate) and cellular alteration (filamentous cells) were noted. With these doses the mycotoxin disturbs various metabolisms : DNA synthesis (gradual blockage of the specific bacteriophage multiplication and dispersion of nuclear apparatus in giant cells); protein synthesis (decrease of protease secretion (2.0 microgram/ml), thuricin excretion (3.0 microgram/ml) and inhibition of endotoxin formation (4.0 microgram/ml); specific activities (bacterial motility, flagellar arrangment and sporulation were respectively affected with 1.0, 0.5 and 4.0 microgram/ml). The mycotoxin did not affect the metabolism of a resistant mutant isolated in the presence of a lethal dose of aflatoxin B1 (mutation rate : 1.10(8) with 20 microgram/ml). The numerous bacterial responses to aflatoxin B1 indicate that the probable site of its toxic binding may not be restricted to a particular locus on the DNA. On the contrary these observations suggest that there exist many combining affinities of the mycotoxin for intracellular sites or interference with a key-function such as the messenger-RNA synthesis.