A method for isolating maternally influenced sex-linked recessive lethals in Drosophila melanogaster has been outlined. By this procedure we obtained mutations that were lethals in that they killed the homozygous or hemizygous lethal progeny of homozygous lethal-bearing female parents. The lethals were maternally influenced in that these classes of progeny survived from heterozygous lethal-bearing female parents. Apparently the latter female parents provided an egg cytoplasm sufficient for progeny survival, and the former ones did not. These lethals were classified as Type I because the daughters of homozygous lethal-bearing females could be rescued by sperm bearing the wild-type allele. With ethyl methane sulfonate as the mutagen, this class of lethals occurred with a frequency of 0.21% and comprised 0.73% of all sex-linked recessive lethals.