Abstract.1. Laboratory experiments with yellow dung fly larvae,Scatophaga sfercorariaL. were carried out in the hope of clarifying the causal relationship underlying the extreme variation in body size in wild populations.2. First, parents were collected in the field and newly hatched larvae exposed to different density levels under two temperature regimes (15°C and 20°C). Second, heritability experiments were done.3. The results showed that density during the larval stage has a very significant effect on adult size. Male size is more affected than female size and the male larvae spend more time feeding than the female larvae with the result that the males are on average larger. Also, the additive genetic component is of little or no significance, supporting the notion that body size is positively related to fitness.4. Finally, the results are discussed in relation to field data on the effects of size on both male and female reproductive succes