Abstract The size distribution of mutant clones in the progeny of T4D phages treated extracellularly with nitrous acid in order to induce mutations is investigated. The results of several single-burst experiments and plaque-picking experiments are presented. In each case a control using untreated phages was done in order to correct the data for spontaneous mutations, background mutations, and any other mutation source not related to the nitrous acid used as our mutagen. By a procedure described by Barricelli (1965) the mutant clone size distribution is used as a means to discriminate between the following two hypothetic alternatives: (1) the simplex hypothesis, holding that only one of the two DNA strands injected by a T4 phage into its host transmits hereditary information to the progeny; (b) the duplex hypothesis, holding that both injected DNA strands transmit their hereditary information to the progeny. The result is in good agreement with the simplex hypothesis and drastically different from the duplex hypothesis expectation. Furthermore, a set of predictions derived for the hypothetic case that the simplex hypothesis is correct for 80% of the phages and the duplex hypothesis for the remaining 20% of the phages do not fit the data as well as the plain simplex hypothesis predictions. The implications for the heteroduplex interpretation of T4 heterozygotes and the question whether the use of mutagenic agents or radiation may inhibit the transmission of hereditary information by one of the injected DNA strands, are discussed.