The effects of weed density on the phytotoxicity of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] to white mustard (Brassica hirta Moench) and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] to foxtail millet [Setaria italica (L.) Beauv.] were determined in the greenhouse. The fresh weight of both plants was reduced by the herbicide treatments, but as the weed density (plants per pot) increased, the herbicide rate required to cause equivalent fresh weight reductions per pot also increased. White mustard and foxtail millet grown in pots absorbed less 14C-atrazine or 14C-alachlor per plant from the soil at high weed density than low, regardless of herbicide rate. Foxtail millet seedlings grown in petri dishes absorbed less 14C-alachlor per plant from aqueous solution as the number of seedlings per petri dish increased. Data from field experiments in 1977 and 1979 indicated that more alachlor or metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] was required for equal control of a high than low density of forage sorghum [Sorghum bicolor (L.) Moench ‘Rox Orange’] interseeded in maize (Zea mays L.). Rox Orange sorghum was used to simulate shattercane [Sorghum bicolor (L.) Moench], a problem weed in Nebraska. At the higher Rox Orange sorghum seeding rate used, both alachlor and metolachlor treatments decreased Rox Orange sorghum plant populations but not yield.