In a new model of light distribution in field crops a parameter s is the fraction of light passing through unit leaf layer without interception. Radiation profiles measured with solarimeters and photocells give values of s from 0.7 for grasses to 0.4 for species with prostrate leaves. Knowing s, leaf transmission T and leaf-area index L the light distribution in a field crop may be described by a binomial expansion of the form {s+(I-s)T)L. To calculate crop photosynthesis at given light intensity this expansion is combined with two parameters describing the shape of the light-response curve of single leaves. Finally, the assumption that solar radiation varies sinusoidally allows daily total photosynthesis to be estimated from daylength and insolation. The theory predicts about the same potential photosynthesis in a cloudy temperte climate with long days as in a more sunny equatorial climate with short days. When L < 3 photosynthesis increases as s decreases, i.e. as leaves become more prostrate; but when L > 5, photosynthesis increases as s increases, i. e. as leaves become more erect. Assuming that respiration is proportional to leaf area, estimated dry-matter production agrees well with field measurements on sugar-beet, sugar-cane, kale, and subterranean clover. Estimates of maximum gross photosynthesis (for sugar-cane and maize) range from 60 to 9 g m−2 day−1 depending on insolation.