A possible factor for the nonrandom distribution of chemically induced chromatid breaks was tested in the root tips of Tradescantia paludosa and Vicia faba, with mitomycin C which is known to inhibit DNA synthesis. The distribution of the induced chromatid breaks changed with recovery time after treatment. The pattern of the changes was correlated closely with the DNA replicating activity of the chromosomes in the experiments performed by other researchers. This phenomenon was interpreted in terms of the inhibition by mitomycin C of asynchronous DNA synthesis and it was suggested that the changes in the aberration distribution with recovery time may resolve some of the differences in the pattern of the distribution which have been said to exist among different radiomimetic chemicals.