Investigations of smooth crabgrass growth and fenoxaprop-ethyl retention, foliar penetration, translocation, and metabolism were conducted at various soil moisture levels using a polyethylene glycol (PEG) semipermeable membrane system. The activity of fenoxapropethyl was significantly reduced at higher levels of moisture stress and this antagonistic effect was greater with increased duration of water deficit following herbicide application. Fenoxaprop-ethyl spray retention decreased linearly (23% total reduction) as soil matric potential (Ψm) decreased from −0.01 to −0.1 MPa. Foliar penetration and translocation of 14C-fenoxaprop-ethyl applied on the third true leaf were not affected by level or duration of moisture stress. Only 2% of the absorbed radioactivity was translocated out of the treated leaf for each moisture stress level and duration. As the soil Ψm decreased (−0.01 to −1.0 MPa) the relative levels of fenoxaprop-ethyl increased by 76 and 65% after a 48- and 96-h postapplication moisture stress period, respectively. In contrast, fenoxaprop acid decreased by 59 and 44% after 48 and 96 h of moisture stress, respectively. The relative level of fenoxaprop acid was linearly correlated to the antagonistic effect on shoot dry weight. These results suggest that decreased spray retention and, particularly, alterations in fenoxaprop-ethyl metabolism contribute to reduced fenoxaprop-ethyl activity observed in moisture-stressed smooth crabgrass.