Studies were conducted to compare physiological characteristics of linuron-susceptible and -resistant Portulaca oleracea. The susceptible biotype had heavier seed and germinated more rapidly than the resistant biotype. Eight weeks after seeding, fresh and dry weights of susceptible P. oleracea plants were significantly greater than those of resistant plants. Susceptible P. oleracea had a significantly higher CO2 assimilation rate at 30 C, but was similar to that of resistant P. oleracea at 40 C. The susceptible biotype had a significantly higher CO2 assimilation rate at CO2 concentrations ≥ 600 ppm and at incident light levels ≥ 900 µmol m−2 s−1. The susceptible biotype had higher carboxylation and photochemical efficiencies.