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Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] effectively controls many annual weeds in established warm-season range grasses but can damage some of these grasses in the early seedling stage. This study determined the relative atrazine tolerance of seedlings of selected warm-season grasses and evaluated the effectiveness of a greenhouse bioassay for seedling atrazine tolerance in range gasses. Seed of 'Pathfinder' switchgrass [Panicum virgatum L.], 'Nebraska 54' indiangrass [Sorghastrum nutans (L.) Nash], 'Butte' side-oats grama [Bouteloua curtipedula (Michx.) Torr.], an experimental blue grama 'PMK 1483' [B. gracilis (H.B.K.) Lag ex Steud], 'Plains' bluestem (Bothriochloa ischaemum var ischaemum (L.) Keng), 'Caucasian' bluestem [B. caucasica (Trin.) C.E. Hubbard] 'Blaze' little bluestem [Schizachyrium scoparium (Michx.) Nash], and a Nebraska experimental prairie sandreed [Calamovilfa longifolia (Hook) Scribn.] were planted in the greenhouse in a 1:l soil-sand mixture that contained 0,1,2, or 3 mg kg-1 atrazine. Seed lots from one cycle of field selection for atrazine tolerance of indiangrass and side-oats grama were included. A field study was also conducted on a silty clay loam (Typic Argiudoll) and on a loamy sand (Udic Haplustoll) site using little bluestem, blue grama, prairie sandreed, Caucasian bluestem, and Plains bluestem. Survival ranking at 3 mg kg-1 atrazine, in the greenhouse, was: Caucasian bluestem = switchgrass > Plains bluestem > prairie sandreed > indiangrass (check) = indiangrass (cycle 1) > side-oats grama (cycle 1) = side-oats grama (check) = blue grama. Atrazine also reduced height in the surviving seedlings. Height reduction in did not appear to be related to seedling survival. Greenhouse data correlated closely with the loamy sand site because the bioassay used a soil-sand mixture. Excellent stands of Plains and Caucasian bluestem were obtained on the silty clay loam site at all atrazine rates. The bioassay was an effective screen to separate grasses with differing atrazine tolerances.