Phosphoinositide 3-kinases (PI3Ks) have been identified in an evolutionarily diverse range of organisms, including mammals, Drosophila, yeast, plants and Dictyostelium. They are activated by a multitude of extracellular signals and implicated in mitogenesis, differentiation and cell survival, as well as in the control of the cytoskeleton and cell shape. Here we describe the molecular and functional analysis of Drosophila p110 (Dp110). A full-length Dp110 cDNA was isolated and found to encode a protein homologous throughout its length to the class I mammalian PI3Ks p110alpha and p110beta. Overexpression of Dp110 in wing or eye imaginal discs resulted in flies with enlarged wings or eyes respectively. In contrast, overexpression of Dp110 containing a mutation predicted to result in the loss of catalytic activity resulted in smaller wings and eyes. The alterations in wing size result from changes in both cell size and cell number, whereas in the eye only differences in cell size were detected. These data imply a role for Dp110 in growth control during Drosophila development and have implications for the function of class I PI3Ks in other organisms.