1. Discs of the foreleg ofCulex pipiens transplanted from the fourth instar of larvae into larval hosts of the same age differentiate normal adult organs. 2. Experiments in fragmentation indicate the position of the primordia of individual parts of the leg in the disc. In addition to this certain structural elements can be located within parts of the leg. 3. Among the majority of host larvae the operation results in a prolongation of the fourth larval instar. In whole discs as well as in disc fragments the sensillae often increase in number, or two groups of sensillae are formed. 4. The implanted disc fragments, after a prolonged presence in the larval host, may regenerate other regions of the disc. 5. Such new formations are the result of agenuine regeneration. The following reasons exclude the assumption of region-specific transdeterminations (HADORN, 1966): a) Structures which are not initially included in the potencies of the cultivated disc fragments (allotypic differentiations -HADORN, 1965a) could not be found. b) Regeneration occurs exclusively in proximo-distal direction. 6. Unlike cultures of the same kind inDrosophila, discs implanted into adult females show no extreme proliferative growth even after a stay in the host of several days. 7. In young adult hosts (0-13 h) several of the implanted discs are caused to form cuticular structures, or, in addition to this, imaginal bristles, trichomes and scales. Discs transplanted into older adult hosts (3 days) retain their larval character. 8. Only a small number of the discs implanted into adult hosts for 22 days are capable of differentiation of imaginal structures after retransplantation into host larvae. The other discs are either reduced inside the host larva or they only contain necrotic tissue.