Nuclear DNA components of Ph. vulgaris were preparatively separated by equilibrium ultracentrifugation in Hg++-Cs2SO4 density gradient (buoyant density of the major component in CsCl density gradient 1.694 g/cm3., satellite component--1.703 g/cm3). The properties of individual DNA fractions were investigated. The melting curve of satellite DNA of Ph. vulgaris has biphasic character. The observed heterogeneity of satellite DNA component is of intermolecular nature. This is illustrated by the splitting of unsheared satellite DNA into two components during renaturation, as well as by its behaviour in Hg++-Cs2SO4 density gradient at high rf value. The width of satellite DNA reassociation curve covers three decades of Cot. The length of the major repeating sequences of the satellite component is close to the length of phage T2 DNA. During chromatography on MAK column satellite DNA elutes earlier than the major component due to its higher GC-content. It is suggested that one of the satellite DNA fractions of Ph. vulgaris contains rRNA genes.