Abstract An account is given of a new mutant,lethal (2) giant discs (l(2)gd; 2–42.7 ± 0.5) ofDrosophila melanogaster, in which the imaginal discs grow to far beyond their normal size during an extended larval period (Figs. 2–6). The extent of the delay in puparium formation in this strain can be modified by changing the culture conditions (Fig. 1). When the larvae are crowded, puparium formation is delayed for up to 9 days, and during this time the imaginal discs grow extensively and acquire an abnormal morphology. Under noncrowded conditions, the delay in puparium formation is usually less than 1 day, and the imaginal discs are correspondingly less abnormal. The time of death (effective lethal phase) is also dependent on culture conditions. Alll(2)gd homozygotes form puparia and pupal cuticle, but there are various defects in eversion of the discs. When the larvae are grown under crowded conditions, death follows shortly after disc eversion; hence no adult cuticle is secreted. In noncrowded cultures, however, about half of the mutant animals are able to produce some adult structures (Figs. 7 and 8). There are characteristic defects in the adult structures produced in these animals, which are also produced byl(2)gd imaginal discs transplanted into wild-type host larvae for metamorphosis (Figs. 9–12). The cuticular hair patterns are normal, but the number of bristles is reduced to about 20% of the number in wild type, and the number of bracts and of sensilla is reduced even more. Those bristles which are produced are usually normal in appearance. Normal aristae are produced, but the ungius is usually missing from the claw organ. Tarsi produced by the leg disc have only two segments instead of five, and appear to have excessive amounts of intersegmental membrane (Fig. 8). In certain imaginal discs, the extensive growth during the lengthened larval period seems to occur preferentially in particular regions. This is reflected in the size of the corresponding structures produced after metamorphosis in wild-type hosts (Fig. 11). In the second and third leg discs, the region of maximal growth is in the thoracic anlage, and this growth is followed by the appearance of a secondary involution and folding in this region (Fig. 4). Leg discs which contain such secondary folds give rise to duplicated legs after metamorphosis in a wild-type host (Figs. 13 and 14), hence the new involution and folding corresponds with a real duplication of anlagen in the disc. The details of the duplication process are discussed; it is suggested that the production of these secondary anlagen resembles the normal development of the leg disc more than it resembles other examples of pattern duplication in imaginal discs. There appears to be a population of cells in the leg disc ofl(2)gd which have special regulative properties. l(2)gd tissue has the capacity for transdetermination, although the frequency is lower than in wild-type tissue.