Concerns have arisen about the possible effects of herbicide contamination in aquatic ecosystems. Crop herbicides are introduced into the aquatic environment both inadvertently through runoff events and intentionally through the use of those registered for use in waterways. Acetochlor and atrazine are two agricultural crop herbicides that have often been reported to contaminate waters. Diquat and fluridone are both registered aquatic management herbicides. In this study, a mammalian in vitro cell cytotoxicity assay was used to evaluate the cytotoxicity of these four commonly used herbicides. The ranked order of the cytotoxicity was: diquat (C(1/2) = 0.036 mM +/- 0.011) > acetochlor (C(1/2) = 0.060 mM +/- 0.010) > fluridone (C(1/2) = 0.172 mM +/- 0.029) atrazine (C(1/2) = 0.581 mM +/- 0.050). In addition, flow cytometric analysis was conducted on CHO cells to investigate the potential impact of these four herbicides on the cell cycle. Acetochlor and diquat had the greatest impact on the cell cycle. Acetochor exposure resulted in a decreased number of cells in the G1 phase of the cell cycle, whereas diquat exposure resulted in a decreased number of cells in both the G1 and G2 phases. Both atrazine and fluridone resulted in a decrease in cells in the G2 phase. The agricultural crop herbicides and aquatic management herbicides gave similar results in cytotoxicity and in the cell-cycle assay at the end points tested.