Abstract The range of usefulness of the high-speed equilibrium centrifugation method of Yphantis (1964) has been extended to measure the molecular weight of Escherichia coli phage T7. Values for \ gn of phages T7, T5 and T4 were obtained by pycnometry; the phage concentrations were determined by measuring nitrogen and phosphorus contents, and the amino-acid and nucleotide composition. The \ gn values are: T7, 0.639; T5, 0.658 and T4, 0.618. From the nitrogen and phosphorus measurements, the percentage DNA in each phage type has been calculated. These values are: T7, 51.2; T5, 61.7 and T4, 54.9. The molecular weights of T7 phage and T7 DNA are 49.4 and 25.3 million daltons, respectively. In the following paper (Dubin, Benedek, Bancroft & Freifelder, 1970), values are reported of the molecular weights of T7, T5 and T4, determined by another technique. The values yielded by the two techniques for the molecular weight of T7 are in excellent agreement. Discussion of the general problem of molecular weight measurement is given in Freifelder (1970). ‡