In studies conducted from 1991 through 1994, researchers investigated the effects of several crop rotations and herbicide programs on crop yield and populations of common lambsquarters, common ragweed, Amaranthus spp., and jimsonweed at two sites. Crop rotations included continuous corn, continuous soybean, corn–soybean, and corn–tomato–soybean, and herbicide programs were the split-plots and included continuous use of acetolactate synthase (ALS)–inhibitor herbicides, continuous use of non–ALS-inhibitor herbicides, annual rotations between ALS- and non–ALS-inhibitor herbicides, combinations of ALS- and non–ALS-inhibitor herbicides in the same year, and no herbicide. Weed control and weed populations generally were affected by an interaction between crop rotations and herbicide programs. After 4 yr, common lambsquarters control was lowest, and populations were highest where fomesafen was used alone for four consecutive years or in rotation with other herbicides. Although common ragweed populations were low at site 2, control at both sites was generally lowest from treatments that included only ALS-inhibitor herbicides. Common ragweed populations were highest at site 1 in 1992 and 1993 following continuous applications of ALS-inhibitor herbicides. Jimsonweed populations were also low at site 2, but control at site 1 in tomato was low. Jimsonweed control from fomesafen and the combination of butylate plus atrazine in soybean and corn, respectively, was variable. Amaranthus spp. populations decreased as the study progressed, and in 1993, control was over 90% from all treatments, except in the case of the treatment combining butylate plus atrazine. Corn and soybean yields varied with year and site, and yields of these crops and tomato were related to rainfall and weed control.