The study of the different phenotypic manifestaions of l(1)ts403 mutation in heterozygotes shown that this mutation is recessive by the following characters: temporal disturbance of heat shock (HS) induced protein synthesis, decreased survivability of imago females and their fertility under HS. This mutation shows incomplete dominance by the criterion of increased frequency of the sex chromosomes loss and nondisjunction in heated females meiosis. Analysis of the genotype of the non-regular XXY females (appearing as a result of HS-induced X-chromosomes nondisjunction in heterozygous females y cv l(1)ts403 f car / l(1)ts403) shown that chromosomes nondisjunction in mutant females is induced, most probably, in first meiotic division. In the most case non- regular XXY females carried non-recombinant X-chromosomes, even though we observed also XXY females with recombinant chromosomes. By the analysis of somatic mosaicism frequency we have shown that HS effects on nondisjunction of the sex chromosomes in female meiosis but not on chromatids nondisjunction in mitosis in strain with l(1)ts403. We have shown significant differencial thermosensitivity in ontogenesis for mutant l(1)ts403 strain: the early embryons (lacking HS-induced HSPs synthesis in wild type embryons) is the most thermosensitive.