Malignant neoplasms that develop in 12 recessive-lethal, larval mutants of Drosophila melanogaster are discussed. These mutations affect the adult optic neuroblasts and ganglion-mother cells in the larval brain, the imaginal discs, and the hematopoietic organs. The malignant neoplasms exhibit fast, autonomous growth, loss of the capacity for differentiation, increased mobility and invasiveness, lethality in situ and after transplantation, and histological, fine structural, and karyotypic abnormalities. Intermediate neoplasms are also found. These combine both benign and malignant qualities. They grow in a noninvasive, compact fashion, typical of benign tumors, yet they also exhibit malignant qualities such as fast, autonomous, and lethal growth, loss of differentiation capacity, changes in cellular morphology, and lethal growth after transplantation into wild-type hosts. Thus Drosophila and vertebrate neoplasms show striking similarities.