Hyphomicrobium bacteriophage Hyphi30 and its nucleic acid were studied to determine some of their biochemical and biophysical properties. The molecular weight of the phage is 55.4 x 10(6), and its buoyant density is 1.508 g/ml. The nucleic acid of Hyphi30 is linear, double-stranded DNA with a molecular weight of 29.7 x 10(6). The guanine-plus-cytosine content of the DNA was 62% as determined from its melting temperature and buoyant density.