Conjugated linoleic acid (CLA) is an unusual fatty acid produced by fermentative bacteria in the rumen of ruminant mammals. Positive biological effects, including anticarcinogenic, antiatherogenic, and immune enhancing effects, have been observed in mammals fed CLA-enriched diets. Little is known of the biological effects of dietary CLA on insects, and nothing is known of the dietary CLA effects on the fatty acid profile of an insect. In this study, we examined the effects of a CLA or safflower oil-enriched meridic diet at several concentrations on European corn borer, Ostrinia nubilalis (Hübner), survival, development, fatty acid profiles, and fecundity. The fatty acid profiles of pupal and adult tissues as well as eggs from adults fed CLA-enriched diets as larvae were studied. Control insects were fed the meridic diet with the solvent carrier added. We hypothesized a CLA-enriched diet, but not a safflower oil-enriched diet, would decrease survival, alter fatty acid profiles, and decrease fecundity. Larvae fed the CLA-enriched diet developed more slowly than did larvae fed the safflower oil-enriched diet or the control diet. Pupal mass was not affected by any of the treatments. Survival was decreased greatly in larvae fed the CLA-enriched diet. Saturated fatty acids increased proportionately, whereas polyunsaturated and monounsaturated fatty acids decreased proportionately in both pupal and adult tissues. Fecundity was not affected by any of the treatments.