Embryos of Drosophila melanogaster, 6-8 h after oviposition, were dissociated and the cells cultured in vitro. Besides larval cell types, imaginal disc cells, assembled and growing in bloated monolayered vesicles, were obtained. The cells of these vesicles become competent to differentiate adult structures when treated with alpha-ecdysone or ecdysterone in vitro. Recognizable patterns of the adult fly are not formed though. If metamorphosis of imaginal cell vesicles from in vitro-cultures is induced in vivo by transplantation into host larvae of various ages within the third larval instar, recognizable patterns can differentiate provided the host larva does not metamorphose prior to 2 days after transplantation. The frequency of specific patterns in the implants can be increased by providing 9 days of culture in vivo (adult host flies) before metamorphosis. Passage through the third larval instar is not essential for these cells to produce identifiable patterns since culture in adult flies alone can achieve this. The quality of the differentiated pattern is not correlated with the extent of cell proliferation in the cultured tissues. The problem of pattern specification in vitro and in vivo is discussed.