Marmur, J. (Brandeis University, Waltham, Mass.), E. Seaman, and J. Levine. Interspecific transformation in Bacillus. J. Bacteriol. 85:461-467. 1963.-Deoxyribonucleic acids (DNA) from various species of the taxonomic group Bacillaceae were examined for base composition, ability to carry out interspecific transformation, and formation of molecular hybrids in vitro. The minimal requirement for genetic compatibility among different species and for DNA interaction (both reflecting base sequence homologies) is the similarity of the guanine plus cytosine contents of the DNA. The close correlation between the ability of DNA to be competent in interspecific transformation and to form hybrid molecules on denaturation and annealing provided a rational approach to the study of genetic relationship among organisms for which no genetic exchange has yet been demonstrated. Any or all of the criteria (base composition of DNA, transformation, molecular hybrid formation) can be used as tools in the taxonomic assessment of closely related microorganisms.