The soybean cultivars differentially sensitive to 'Kwangkyo' and 'Hood' are the bipyridylium herbicide paraquat (1,l'-dimethyl-4,4'-bipyridinium ion). This was confirmed by visible injury observations as well as by measurements of desiccation ratings, chlorophyll content, and chlorophyll fluorescence tracings of fully expanded first trifoliolate leaves of these two cultivars after exposure to a wide range of paraquat concentrations. The margin of this intraspecific differential tolerance to paraquat was narrow and the ratio of the paraquat concentrations causing 50% injury to the tolerant Kwangkyo and to the susceptible Hood (approximate tolerance factor) was found to be 10. Paraquat at 1 μM or higher inhibited rapidly the CO₂ fixation capacity of leaf mesophyll cells, isolated enzymatically from both cultivars. This result shows that the differential response does not result from any differences at the site of paraquat action in chloroplast membranes. / Ph. D.