The first lethal found by Miss Rawls was in a stock that had been bred for about 3 years. While there was no _a priori_ reason that could be given to support the view that lethal mutations would occur more frequently among flies inbred in confinement, nevertheless a hundred females from each of several newly caught and from each of several confined stocks were examined for lethals (Stark, 1915). No lethals were found among the wild stocks, but 4 were found among the confined stocks. Whether this difference is significant is perhaps open to question. The first lethal was found in January 1913, in a stock that had been caught at Falmouth, Massachusetts, in 1911, and had been inbred for 18 months, _i.e._, for about 50 generations. This lethal, lethal _sa_, was recessive and behaved like the former lethals, being transmitted by half the females and causing the death of half the sons. The position of this lethal to the X chromosome was found as follows, by means of the cross-over value white lethal _sa_. Lethal-bearing females were mated to white males and the lethal-bearing daughters were again mated to white males. The white sons (894) were non-cross-overs and the red sons (256) were cross-overs. The percentage of crossing-over {64} is 22.2. A correction of 0.4 unit should be added for double crossing-over, indicating that the locus is 22.6 units from white, or at 23.7. When the work on lethal _sa_ had been continued for 3 months, the second lethal, lethal _sb_, was found (April 1913) to be present in a female which was already heterozygous for lethal _sa_. It is probable that this second lethal arose as a mutation in the father, and that a sperm whose X carried lethal _sb_ fertilized an egg whose X carried lethal _sa_. As in the cases of lethals 1 and 1_a_ and lethals 3 and 3_a_, this lethal, lethal _sb_, was discovered from the fact that only a very few sons were produced, there being 82 daughters and only 3 sons. If, as in the other cases, the number of daughters is taken as the number of non-cross-overs and twice the number of sons as the cross-overs, it is found that the two lethals are about 7 units apart. Since the two lethals were in different X chromosomes, all the daughters should receive one or the other lethal, except in those few cases in which crossing over had taken place. Of the daughters 19 were tested and every one was found to carry a lethal. Again, if the cross-over values of the lethals with some other character, such as white eyes, be found and plotted, the curve should show two modes corresponding to the two lethals. This test was applied, but the curve failed to show two modes clearly,[7] the two lethals being too close together to be differentiated by the small number of determinations that were made. It seems probable that lethal _sa_ and lethal _sb_ are about 5 units apart.