Concentrations of atrazine and metolachlor and stereoisomer fractions (SF = herbicidally active/total stereoisomers) of metolachlor were determined in 101 surface water samples collected from the five Laurentian Great Lakes in 2005-2006. Geometric mean (GM) concentrations of atrazine ranged from 5.5 to 61 ng L(-1), decreasing from lakes Ontario approximately Michigan approximately Erie > Huron > Superior, while metolachlor concentrations ranged from 0.28 to 14 ng L(-1) and showed similar trends among the lakes. Median SFs ranged from 0.527 (Superior) to 0.844 (Erie) with an overall value of 0.708, and were significantly different among the Great Lakes (p < 0.05), except for Michigan vs Huron and Michigan vs Ontario. The SF in Erie was closest to that of the dominant product in use, S-metolachlor (SF = 0.880), while Superior showed an SF similar to that of racemic metolachlor (SF = 0.500). The median SFs in lakes Ontario, Huron and Erie were significantly lower than the median SF in Ontario stream samples collected in 2006-2007. The lower SFs in lakes suggest in-lake stereoselective processing of metolachlor or hold-up of older racemic metolachlor residues.