A set of lines of Drosophila melanogaster was generated, all derived from a common inbred base stock and each with a single new P element insert on the second or the 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 nature of the construct and controls prevented deduction of the direction of mutant effects. There was a strong correlation between the effects of the inserts on the thorax length of males and of females, and there were also highly significant sex-specific effects. Effects of the inserts on wing area were highly correlated with those on thorax length, suggesting an effect on size of the imaginal disc from which both these structures are derived. Inserts affected both cell size and cell number in the wing, apparently independently. Pleiotropic effects of inserts affecting body size on viability and bristle number 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 fitness-related traits.