In the animal kingdom, carotenoids are usually absorbed from dietary sources and transported to target tissues. Despite their general importance, the uptake mechanism is still poorly understood. Here we report the " red crop" phenomenon, an accumulation of α- and β-carotene in crystalline inclusions in the enlarged foregut of the polyphagous Spodoptera larvae feeding on some potentially toxic plant leaves. The carotene crystals give the insect foregut a distinctive orange-red color. The crystals are embedded in a homogenous lawn of the bacterium Enterococcus casseliflavus, but the carotene seems to be selectively taken from the food plant. Caterpillars which fail to develop these carotene crystals exhibit a high mortality or fail to develop to adulthood. The crystallization of carotene and the enlargement of the foregut thus appears to manifest a multiple-step physiological adaptation of the insects to toxic food plants.