Conservation tillage is frequently used for corn (Zea Mays L.) and soybean [Glycine max (L.) Merr] production to reduce soil loss, improve soil quality, and maintain eligibility for commodity support payments. The residual herbicides normally used in the production of these crops, however, are often detected in surface runoff, rivers, and reservoirs at concentrations that exceed drinking water standards. With the advent of transgenic, herbicide-tolerant crops, such as glufosinate-tolerant corn [2-amino-4-(hydroxy-methylphosphinyl) butanoic acid] (Liberty-Linked) and glyphosate-tolerant [N-(phosphonomethyl)-glycine] soybean (Roundup Ready), this concern might be reduced by replacing residual herbicides with short-half life, strongly sorbed, contact herbicides. Therefore, we applied both herbicide types at the maximum recommended rates to 2 chisel-plowed and 2 no-till watersheds in a 2-yr corn/soybean rotation and at half rates to 3 reduced-input, disked watersheds in a 3-yr corn/soybean/wheat-red clover (Triticum aestivum L. -Trifolium pratense L.) rotation and monitored herbicide losses in surface runoff on an individual event basis for 4 crop years (1998-2001). In corn years, average annual loss of glufosinate (0.06%) was less than the losses of atrazine-0.37% (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4- diamine), alachlor-0.07% [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide], and linuron-0.15% (3-3,4-dichlorophenyl-1-methoxy-1-methylurea), the residual herbicides it can replace. The maximum, annual, flow-weighted average concentration of glufosinate was 3.5 µg/L, whereas the maximum, annual, flow-weighted average concentrations of atrazine (31.5 µg/L) and alachlor (9.8 µg/L) were substantially higher and exceeded the Maximum Contaminant Levels (MCL) of 3 µg/L for atrazine and 2 µg/L for alachlor. The maximum annual flow-weighted concentration of linuron, which does not have an established MCL, was 8.2 µg/L. Regardless of tillage system, the annual, flow-weighted average atrazine and alachlor concentrations exceeded their MCLs in at least one crop year. Similarly, during soybean years, average annual loss of glyphosate was 0.07% compared to a metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] loss of 0.48% and an alachlor loss of 0.12%, the residual herbicides it can replace. Maximum, annual, flow-weighted concentration of glyphosate was 9.2 µg/L, whereas maximum alachlor and metribuzin concentrations were 44.5 and 9.5 µg/L, respectively. Thus, while growing transgenic corn and soybean and replacing residual herbicides with glufosinate and glyphosate did not eliminate herbicide losses in surface runoff, transport and concentrations where substantially reduced in some instances. Moreover, glufosinate and glyphosate are of lower toxicity than the residual herbicides they can replace and their presence in surface water should be of less environmental concern.