The first case of field-evolved paraquat resistance in a population of Lolium rigidum Gaud. (from the Western Cape, South Africa) was confirmed and the mechanism of resistance investigated. The LD50 for the resistant population (R) was 404 g ha(-1), some 14 times greater than for the herbicide-susceptible (S) population (30 g ha(-1)). In addition, the R population was found to be more resistant to paraquat when kept at low temperature (15degreesC) than when kept at 30degreesC after paraquat treatment. The R population is normally affected by herbicides with other modes of action. No differences were found in the interaction of paraquat with Photosystem I in thylakoids isolated from the R and the S populations. Constitutive levels of the antioxidative enzymes superoxide dismutase (SOD) and ascorbate peroxidase (APX) did not differ significantly between the two populations and these enzymes responded similarly to paraquat treatment. When [C-14] paraquat was applied as droplets to intact plant leaves, similar leaf uptake of [C-14] paraquat occurred in the R and S populations. However, quantification data and phosphor imaging revealed restricted translocation of [C-14] paraquat to untreated leaves in the R compared to S population. The results of this study with this resistant L. rigidum population from South Africa resemble those found in R biotypes of Hordeum spp. from Australia. The resistance is suggested to be primarily due to sequestration of paraquat, limiting its translocation within the plants. The exact site and mechanism of paraquat sequestration remains to be determined.