Effects of food quality on mortality and developmental rate of a Finnish population ofDiprion piniwere studied in the laboratory. The larvae were reared both on Scots pine needles taken from a site with a heavy pollution load, and from an area with no industry in the vicinity. Individuals reared on polluted needles tended to develop slightly at a slower rate and to hatch later than those reared on unpolluted needles. Cocoon mortality was higher in the group fed with polluted needles. Cocoon weight was not related to the hatching time within the groups.