Sixteen field experiments were conducted over a three-year period (2003--2005) at two Ontario locations to evaluate efficacy of pre-emergence and post-emergence herbicides for the control of waterhemp in corn and soybeans. The waterhemp biotype was resistant to both the acetolactate synthase inhibitors and photosystem II inhibitors at one location and resistant to only the acetolactate synthase inhibitors at the second location. Waterhemp control was dependent on the resistance pattern of the waterhemp and the environmental conditions at each location. In corn, isoxaflutole, 's'-metolachlor/atrazine, mesotrione, or 'S'-metolachlor/atrazine plus mesotrione applied preemergence, and dicamba, dicamba/atrazine, and mesotrione plus atrazine applied postemergence all resulted in greater than 90% control of waterhemp. In soybeans, 's'-metolachlor plus metribuzin applied pre-emergence, and acifluorfen, fomesafen, imazamox plus fomesafen, and glyphosate applied post-emergence, provided greater than 87% waterhemp control in three out of four site-years. Two greenhouse experiments found triazine-resistant waterhemp produced less seed than triazine-susceptible waterhemp.