The density of the cytoplasmic DNA of two strains of "petite" mutants of yeast, obtained by treatment with acriflavin and with ultraviolet light, was examined in cesium chloride density-gradient centrifugation and in all cases appeared to be less than that of the wild type. A cytoplasmic respiratory-deficient strain, treated with additional acriflavin, can show a further shift of the position of the satellite band, always in the direction of reduction of density. Also, from the ρ+× ρ-cross, ρ-strains can be recovered in which the density of the satellite DNA is different from the density of the parent ρ-strain. This finding suggests the existence of recombination in cytoplasmic DNA moleciules.