The growth of healthy green crops is proportional to the solar radiation intercepted and the fraction of light intercepted by the cropfis the major source of variation in crop yields. A two band spectrophotometer was used to measure solar radiation reflected from crops of sugar beet in the red (600-660 nm) and the near-infrared (780-940 nm) bands. The ratio of infrared/red reflected fluxes increased with the percentage of the ground covered by healthy green leaves and was used to estimatefOn the assumption that the efficiency with which crops convert radiant energy to biomass ϵ is constant, such measurements were used to determine future rates of growth and predicted the yield of an independently grown crop to within 6 per cent. Preliminary evidence suggests that spectral ratios may also be sensitive to changes in ϵ and the extension of these techniques to crops limited by disease or environmental stress is discussed.