Differences between light reflectance from leaves of cotton (Gossypiurn hirsutum) plants grown with a low- or no-nematode (Rotylenchulus reniformis) population (nonstressed), and from leaves grown with a high nematode population (stressed) were measured in field and greenhouse experiments. Reflectance was measured spectrophotometrically in the laboratory on single leaves and spectroradiometrically in the field on plant canopies. Nematode-stressed cotton plants were stunted with fewer, smaller, and darker-green leaves than nonstressed plants. Over the 0.5- to 2.5-/microm waveband, stressed leaves had lower reflectance than nonstressed leaves of the same chronological age for both field- and greenhouse-grown plants. Reflectance differences between stressed and nonstressed leaves in the visible (0.5 to 0.75 microm), near-infrared (0.75 to 1.35 mum) and infrared water absorption (1.35 to 2.5 microm) regions were primarily caused by differences in leaf chlorophyll concentration, mesophyll structure, and water content, respectively. Results indicate the potential for remotely sensing nematode-infested plants to distinguish them from normal plants.