M1 Drosophila melanogaster laboratory strains fall into two main classes (inducer and reactive) on the basis of a quite specific kind of partial intersterility. All strains from the wild are inducer. The genetic element responsible for the inducer condition (I factor) is found on most of the inducer strains chromosomes: a chromosome carrying the/factor (i + chromosome) is characterized by its ability to give rise, when introduced by a paternal gamete into a reactive oocyte, to a female showing reduced fertility; chromosomes of inducer strains which do not carry I are denoted i °. Another result of the nucleocytoplasmic interaction between the I factor and the reactive cytoplasm, is the ability of i ° and reactive originating chromosomes to irreversibly acquire I, independently of any classical genetic recombination. However, this so-called chromosomal contamination does not occur in inducer i+/i ° heterozygous females. Using such females, mapping experiments showed that I can be located at a single locus on the w ct .f X chromosome and a single locus on the b bw second chromosome (approximate locations: 1-33 and 2-61) and at probably two independent loci on the see third chromosomes. Abnormal second chromosome transmission frequencies were observed; they do not seem to be related to previously reported segregation distortion phenomena.