In a previous communnication' it was suggested that the chromosome of each particle of phage T4 is terminally redundant, and that after several rounds of replication (following infection), progeny chromosomes arise which have circularly permuted genetic sequences. Among the niembers of a population of phage particles, the ends of the chromosomes would be randomly distributed over the genome, and a phage particle carryinig different alleles at one of its redundant loci would be heterozygous. Nomura and Benizer2 have reported that heterozygotes for rII deletioni mutants (from mixed infections with r+ phage) occur with about a third of the frequency of those for rII point mutants. It thus seemed likely that heterozygosity in phage T4 is of at least two kinds. One kind may be the result of heterozygosity within the molecule, as was first suggested by Levinthal, whereas another kind may be due to the terminal redundancy described above. Deletion mutants may be unable to form heterozygotes of the internial kind since these would involve violations of duplex complementarity (regardless of their precise molecular structure), whereas point mutants may form heterozygotes of both kinds. The chromosome of a phage particle will be termimnally redundant and heterozygous for marker a provided that (1) the chromosome terminates near marker a and includes marker a, and (2) the last recomnbinatiomnal event, occurring somewhere within the chromosome, has been such that the other end of the chromosome contains marker a+. Terminal-redundancy heterozygotes are formed and lost by the process of recombination; their frequency (as a function of time after infection) will depend on the rate of recombination but not on the rate of replication. In a pool of vegetative chromosomes, the frequency of terminally redundant heterozygotes would be expected to change in time as does the frequency of recombination for very distanitly liniked markers; it would be expected to increase until an equilibrium value was reached and to remain constant after that. Because of the high frequency of recombination in phage T4, onne would expect that the equilibrium value would be reached very early during the latent period. Internal heterozygotes are most probably formed by an event that results in the recombination of markers on either side of the heterozygous region;3 thus, they may well represent the primary products of recombination. They would be expected to disappear upon the semiconservative replication of the chromosome bearing them. The chromosome of a phage particle will be internally heterozygous for marker a provided that (1) a recombinational event leading to internal heterozygosis will have occurred at the site of marker a, and (2) the chromosome is incorporated into a mature particle before it unidergoes replication. Internal heterozygotes are formed by recombination and are lost by replication: their frequency (as a function of