Frequency and localization of spontaneous and induced by high temperature (37 degrees C) recessive lethal mutations in X-chromosome of females belonging to the 1(1) ts 403 strain defective in synthesis of heat-shock proteins (HSP) were studied. No differences in frequencies of both spontaneous and induced lethals between 1(1) ts 403 and control strain were found, thus implying that the disturbances in HSP synthesis have no effect on this process in oocytes of Drosophila melanogaster females. Surprisingly, distribution of spontaneous and induced lethals along the X-chromosome of 1(1) ts 403 strain appeared to be non-random: they primarily are located in its distal portion (1-44 cM of genetic map or in I-II sections of the Bridges cytogenetic map). This correlates with non-random distribution of mobile elements in the X-chromosome of D. melanogaster (Leibovich, 1990).