Mutants of Bacillus subtilis with a heat-sensitive spores have been isolated. Of these mutants, a strain 1508C required simultaneously thiamine and grew normally when thiamine was provided. The mutant have a double requirement for the pyrimidine and thiazole moieties of the vitamin molecule. All the spontaneous revertants isolated from the mutant restores the thiamine deficiency, but does not result in increased heat-resistance of spores, compared with those of wild type.