The relative phytotoxicity of sorgoleone as measured by seed germination and seedling growth of selected crop and weed species and inhibition of photosynthetic oxygen evolution in atrazine-resistant and -susceptible cell cultures of potato (Solanum tuberosum L.) and common groundsel (Senecio vulgaris L.) were investigated. Relatively little or no effect of sorgoleone was observed on radicle elongation at concentrations less than 500 μM. in Petri dish bioassays. Sorgoleone was very phytotoxic to large crabgrass (Digitatia sanguinalis), with a GR50 of 10 μM. for shoot growth in a hydroponic culture bioassay. Inhibition of shoot and root growth of velvetleaf (Abutilon theophrasti) and barnyardgrass (Echinocloa crus-galli) was also observed at higher concentrations ranging from 10 to 200 μM, but ivyleaf morningglory (Ipomea hederacea) was tolerant. Sorgoleone inhibited photosynthetic oxygen evolution in both susceptible and resistant cell cultures of potato and common groundsel, and the effect was similar to that of diuron, a strong inhibitor of PS II electron transport. Chlorophyll fluorescence response to sorgoleone in both resistant and susceptible cell cultures was nearly the same. Grain sorghum (Sorghum bicolor L. Moench) genotypes varied considerably in the amount of sorgoleone produced. Root exudates generally contained 85-90% pure sorgoleone on the basis of HPLC analysis. These data indicate that sorgoleone is phytotoxic at micromolar concentrations, exhibits marked selectivity, and inhibits photosynthetic electron transport similar to diuron.