Laboratory studies were conducted to determine the basis for chlorsulfuron selectivity between Kentucky bluegrass (Poa pratensis L. cv. Kenblue) and tall fescue (Festuca arundinacea Schreb. cv. Rebel). Tall fescue absorbed and translocated more foliar-applied [14C]-labeled chlorsulfuron from the treated leaf than Kentucky bluegrass. The two species absorbed similar amounts of chlorsulfuron from nutrient solution into the roots, but tall fescue translocated more of the absorbed radioactivity to the shoots. Tall fescue metabolized chlorsulfuron in the shoots slightly more slowly than Kentucky bluegrass. Allof these factors apparently contributed to the higher tolerance of Kentucky bluegrass than of tall fescue to chlorsulfuron. Chemical name used: (2-chloro-N-[[4-methoxy-6-methyl-1,3,5 -triazin-2-yl)amino]-carbonyl] benzenesulfonamide) (chlorsulfuron).