The red color of Royal Gala apple (Malus domestics Borkh.) skin increased in intensity following irradiation with ultraviolet (UV) and white light. The enhanced red apple color was due to an increase in anthocyanin concentration and the increase was dose dependent. High-performance liquid chromatography analysis showed that the composition of flavonoids in UV treated and natural red colored apple skins was similar. The red apple skin color further increased after storage at 4C in the dark. During the course of irradiation the enzymatic activities of phenylalanine ammonia lyase (PAL) and chalcone isomerase (CHI) increased 10-to 20-fold. Northern analysis showed an increase in PAL transcripts during the irradiation treatment, suggesting that the increase in PAL enzymatic activity was due to de novo synthesis of the enzyme in apple skin cells.