Third instar larvae of the Oregon R stock were treated for 25 or 30 min with vapour of methyl methanesulphonate (MMS). At various times after the treatment (4, 8 and 12 h), microscope specimens of the nerve ganglia were prepared. At all the times of fixation, only aberrations of a chromatid type were found, but with different frequencies in the two sexes. The females were about 3 times more sensitive than the males to MMS. An analysis of the distribution of the breaks between and within chromosomes showed that they were not localized at random but were clustered in the heterochromatic centromere regions of the X chromosome and the autosomes. The Y chromosome, although entirely heterochromatic, on the other hand, was highly resistant to MMS. However, this phenomenon had only a very weak effect upon the variation with sex of MMS-induced chromosome damage. Among the aberrations induced, although interchanges were normally present, neither intra-exchanges nor triradials were found. In the interchanges, there was a greater tendency for the homologous chromosomes to be involved, and these rejoined symmetrically. The above results are discussed in relation to data previously obtained with X-rays.