We have examined the distribution of proteins in the apico-lateral cell junctions in Drosophila imaginal discs. The subcellular distribution of these proteins in normal and mutant proliferating cells was analyzed with marker antibodies and confocal microscopy. Antibodies to phosphotyrosine (PY), Armadillo (Arm) and Drosophila E-cadherin (DE-cad) as well as FITC phalloidin marking filamentous actin, labeled the site of the adherens junction, whereas antibodies to Discs large (DIg), Fasciclin III (FasIII) and Coracle (Cor) labeled the more basal septate junction. The junctional proteins labeled by these antibodies underwent specific changes in distribution during the cell cycle. We have previously shown that a loss-of-function dlg mutation, which causes neoplastic imaginal disc overgrowth, leads to loss of the septate junctions and the formation of what appear to be ectopic adherens junctions [Woods et al., 1996]. We therefore extended this study to examine the effects of mutations in other genes that also cause imaginal disc overgrowth. Based on staining with PY and DIg antibodies, the apico-lateral junctional complexes appeared normal in tissue from the hyperplastic overgrowth mutants fat, dco, gd and wts. However, imaginal disc tissue from the neoplastic overgrowth mutants dlg and lgl showed abnormal distribution of the junctional markers including a complete loss of apico-basal polarity in loss-of-function dlg mutations. These results support the idea that some of the proteins of apico-lateral junctions are required both for apico-basal cell polarity and for the signalling mechanisms controlling cell proliferation, whereas others are required more specifically in cell-cell signalling.