The maximal chlorophyll contents reached during the growth of each of the first three leaves of Marquis wheat increased linearly with daylength, when the plants were grown from germination under illumination of 1750 ft-c intensity for daily periods of 8, 16, or 24 hours. This effect of daylength was also seen when chlorophyll concentration was expressed on a unit leaf area or fresh weight basis, and was related to increased leaf thickness at the longer daylengths. The increased chlorophyll content and concentration under long daylengths could not be duplicated by extending short daylengths with low-intensity supplementary light. Previous experiments suggest that the control of chlorophyll content by daylength is through the control of leaf thickness by the total amount of daily energy.The ratio of chlorophyllatobin mature leaves did not vary with daylength. There was also no change in the maximal chlorophyll concentration of leaves mature at the time of floral initiation compared with those mature in the vegetative stage of development.