Club females were obtained by breeding F_2 club males to their F_2 long-winged sisters, half of which should be heterozygous for club. {69} 5,352; wild-type [male], 4,181; club [male], 236. The wild-type males include, of course, those club males that have expanded wings (potential clubs). Club females by wild males gave in the F_2 generation (mass cultures): wild-type [female], 1,131; wild-type [male], 897; club [female], 57; club [male], 131. It is noticeable that there were fewer club females than club males, equality being expected, which might appear to indicate that the club condition is more often realized by the male than by the female, but later crosses show that the difference here is not a constant feature of the cross. Long-winged males from club stock (potential clubs) bred to wild females gave in F_2 the following: wild-type [female], 521; wild-type (and potential club) [male], 403; club [male], 82. Club females by club males of club stock gave in F_2: potential club [female], 126; potential club [male], 78; club [female], 95; club [male], 81. These results are from 8 pairs. The high proportion of club is noticeable. Potential club females and males from pure club stock (_i. e._, stock derived originally from a pair of club) gave in F_2 the following: potential club [female], 1,049; potential club [male], 666; club [female], 450; club [male], 453. GENOTYPIC CLUB. Accurate work with the club character was made possible by the discovery of a character that is a constant index of the presence of homozygous club. This character is the absence of the two large bristles (text-fig. H_c_) that are present on each side of the thorax of the wild fly as shown in figure Hb. All club flies are now classified by this character and no attention is paid to whether the wings remain as pads or become expanded. LINKAGE OF CLUB AND VERMILION. The linkage of club and vermilion is shown by the cultures listed in table 51, which were obtained as controls in working with lethal III. The cross-over value is shown in the male classes by the cross-over fraction 276/1463 or 19 per cent. LINKAGE OF YELLOW, CLUB, AND VERMILION. The data just given in table 51 show that club is 19 units from vermilion, but in order to determine in which direction from vermilion it lies, the crossing-over of club to one other gen must be tested. For this test we used yellow, which lies at the extreme left of the chromosome series. At the same time we included vermilion, so that a three-point experiment was made. Females that were (gray) club vermilion were bred to yellow (not-club red) and gave wild-type daughters and club vermilion sons. These inbred gave the results of table 52.