Field experiments were conducted in 1994 and 1995 under sprinkler irrigation at the University of Wyoming Research and Extension Center at Torrington to evaluate the effects of season-long interference and the effects of duration of interference of several common sunflower and green foxtail densities, alone or in combination, on pinto bean yield. Green foxtail densities did not significantly affect pinto bean yield in 1994 and reduced yield only at the highest density in 1995. In contrast, sunflower densities reduced pinto bean yield, except at the lowest density in 1994. Pinto bean yield was reduced as the combined density of green foxtail and sunflower increased. Compared with yield losses from each weed species alone, yield reductions from mixed species were additive in 1994 and at low weed densities in 1995 and less than additive at higher weed densities in 1995. The minimum number of weeds per m of row that will economically reduce pinto bean yield was estimated to be 1.6 to 2.9 for green foxtail and 0.12 to 0.2 for sunflower. Pinto bean yield reduction increased as the duration of green foxtail and sunflower interference increased, whether grown alone or in combination. The maximum duration that green foxtail, sunflower, and green foxtail plus sunflower can interfere with pinto bean before causing economical losses was estimated to be 4.5, 3.2, and 2.5 wk, respectively.