The Drosophila neoplastic tumor suppressor genes (TSGs) coordinately control cell polarity and proliferation in epithelial and neuronal tissues. While a small group of neoplastic TSG mutations have been isolated and their corresponding genes cloned, the regulatory pathways that normally prevent inappropriate growth remain unclear. Identification of additional neoplastic TSGs may provide insight into this question. We report here the design of an efficient screen for isolating neoplastic TSG mutations utilizing genetically mosaic larvae. This screen is based on a defective pupation phenotype seen when a single pair of imaginal discs is homozygous for a neoplastic TSG mutation, which suggests that continuously proliferating cells can interfere with metamorphosis. Execution of this screen on two chromosome arms led to the identification of mutations in at least seven new neoplastic TSGs. The isolation of additional loci that affect hyperplastic as well as neoplastic growth indicates the utility of this screening strategy for studying epithelial growth control.