Eye-antennal and ventral mesothoracic (midleg) disks of 7 1/2 days old, mature larvae ofD. hydei, homozygous for the lethal genelpl (lethal-polyploid), have been transplanted into 7 1/2 or 4 1/2 days old larval wildtype hosts.The morphology oflpl disks shows a variation of damage ranging from complete absence or strongly reduced anlagen to normal sized disks. Transplanted disks are able to undergo metamorphosis and to differentiate adult structures. The inventory of structures formed varies individually and is always less complete than differentiations obtained from wildtype controls, but the topographical arrangement appears to be normal. Even after a prolonged stay in the larval hosts, proliferation processes are rare, and transdetermination was not observed.The results are discussed, and it is suggested that the mutant effect which appears to be cell autonomous but not fully penetrant causes a mitotic disturbance which, at a low incidence, affects randomly the cell divisions of mitotically active tissues throughout larval life.