Spectral reflectances were measured by tri-band densitometry of aerial color-infrared photographs of soybean (Glycine mas fields that had been affected by sulfur dioside (SO2) emissions from large, coal-fired power plants in northwestern Alabama and western Tennessee. The photographs were photometriacally calibrated. Results indicate that, at very light levels of foliar injury, the infrared-to-red reflectance ration decreased with increasing injury. This behavior was in accordance with theory. However, at moderate and severe levels of injury, the ratio increased with injury. The best indicator of crop yield was green band reflectance, but the red and infrared bands were nearly as good. The yield variable actually increased with the level of injury, apparently because of field-to-field variations in canopy density.