HOW THE FACTORS ARE LOCATED IN THE CHROMOSOMES. A character is in the first chromosome if it is transmitted by the grandfather to half of his grandsons, while, in the reciprocal cross, the mother transmits her character to all her sons (criss-cross inheritance) and to half of her granddaughters and to half of her grandsons; in other words, if the factor that differentiates the character has the same distribution as the X chromosome. If, however, a new mutant type does not show this sex-linked inheritance, its chromosome is determined by taking advantage of the fact that in _Drosophila_ there is no crossing-over in the male between factors in the same chromosome. For instance, if a new mutant type is found not to be sex-linked, its group is determined by the following tests: It is crossed to black, whose factor is known to be in the second chromosome, and to pink, whose factor lies in the third chromosome. If the factor of the new form should happen to be in the second chromosome, then, in the cross with black, no double recessive can appear, so that the F_{2} proportion is 2:1:1:0; but with pink, the mutant type should give the proportion 9:3:3:1, typical of free assortment. If, however, the factor of the new form is in the third chromosome, then, when crossed to black, the double recessive and the 9:3:3:1 proportion appear in F_{2}. But when crossed to pink no double recessive appears in F_{2}, and the proportion 2:1:1:0 occurs. If these tests show that the new mutant does not belong to either the second or third chromosome, that is, if both with black and with pink the 9:3:3:1 ratio is obtained, then by exclusion the factor lies in the fourth chromosome, in which as yet only two factors have been found. We propose to give in a series of papers an account of the mutant races of _Drosophila_ and the linkage shown in their inheritance. In this paper we shall consider only the members of the first chromosome, describing a large number of new mutants with their linkage relations and summarizing to date all the linkage data relating to the first chromosome. In later papers we propose to consider the members of the second, third, and fourth chromosomes. The list at the top of page 21 gives the names of the factors dealt with in this paper. They stand in the order of their discovery, the mutant forms reported here for the first time being starred.