The results of drought stress on a crop of soybean have been observed using laser-induced fluorescence kinetics of individual leaves in two wavelength bands. Thisin vivochlorophyll fluorescence is particularly useful for early stress detection in plants recorded in the field. In the context of remote sensing, measurements on leaves sampled from different levels in the canopy and a mathematical simulation have shown the importance of the ratio of fluorescence peaks in the wavelength bands 690 and 730 nm. This index is not very sensitive to the leaf area and structure of the vegetation.