A set of Drosophila melanogaster was generated, all derived from a common isogenic base stock and each with a single new P element insert on the second or third chromosome. The lines were scored for their body size, measured as thorax length. P inserts were associated with highly significant effects on body size, although the genotypes of the construct and of the control prevented deduction of the direction of mutant effects. In addition to mutant effects on the thorax length of both sexes, there were also highly significant sex-specific effects. Pleiotropic effects of inserts affecting body size on viability and bristle number, as ascertained in a separate study of these lines (Lyman et al., 1996), were weak. Insertional mutagenesis is potentially a powerful tool for investigating the genes involved in size-control in Drosophila, but the technique requires fine tuning for use on polygenic and fitness-related traits.