In the lecanoid chromosomal system, as exemplified by the mealy bug, Planococcus citri (Risso), one chromosome set becomes heterochromatic during embryogeny of the male and is maintained as such during development. At spermatogenesis, the first division is equational for both the euchromatic and heterochromatic chromosomes which are segregated from each other in the second; only the euchromatic derivatives form sperm. After paternal irradiation, the induced aberrations appear in the heterochromatic set of the male embryos while they occur in the euchromatic set after maternal treatment. After paternal irradiation of doses up to 30000 rep, dominant lethality is induced in daughters but not in sons. There is thus a clear picture of the failure of damage to the heterochromatic set to produce a detectable genetic effect. These results do not rule out a secondary genetic or physiological function of the heterochromatic set. No breakage-fusion-bridge cycles, which would complicate the results whether the heterochromatic set were inert or not, were observed in the cytological studies after low-dosage treatments. After maternal treatment, both sexes diminish with increasing dosage but the results are complicated by the effect of maternal age on the sequence in which the two sexes of offspring occur during oviposition (sexualmore » dichronism) as well as on total progeny. At high-dosage paternal treatments, 60, 90, and 120,000 rep, the number of sons is drastically reduced while the number of daughters is about 40% of the controls. (P.C.H.)« less