Cells of Chlamydomonas eugametos Moewus, sex minus, were treated with tritiated myleran and a correlation was established between the mutation frequency (loss of resistance to streptomycin) and the tritium content of the isolated DNA. It was concluded that the resistance to streptomycin is due to a single DNA locus per cell; this is particularly interesting in view of the fact that these algae cells are haploid. It has been supposed that the observed mutations were due nearly exclusively to the cross-linking of the complementary strands of the DNA molecule carrying the competent locus. The size of this DNA molecule was provisionally calculated to be 3,600,000 or 2,900,000 a.m.u.;† these results are derived from two separate sets of experiments.