Cells of Bacillus subtilis strains 168 and W23 deprived of an amino acid or a base on a given solid medium were found to develop competence. We describe a rapid and simple method of genetic transformation of this organism consisting in spreading a sample containing 1 microgram DNA and 10(7) exponentially growing cells of an auxotrophic mutant onto plates devoid of the required amino acid or base. After overnight incubation, about 100-200 prototrophic transformants per plate were obtained, i.e. a frequency of about 10(-5), as compared to 10(-4) routinely obtained by the method of transformation in liquid medium with frozen competent cells. Plasmids and other chromosomal or plasmid-borne markers, which cannot be directly selected for, were transferred by congression. The dependence of the transformation efficiency on cell density, medium richness, incubation time and the nature of transforming DNA was investigated. We conclude that the development of competence accompanies amino acid or base starvation of cells under appropriate physiological conditions.