Wild radish is a major weed of field crops in southern Australia. The effects of various densities of wild radish and wheat on the growth and reproductive output of each other were investigated in field studies in 2003 and 2004. The experiments were established as a factorial combination of wheat (0, 100, 200, and 400 plants m−2) and wild radish (0, 15, 30, and 60 plants m−2) densities. The effect of wild radish density on wheat yield loss and wild radish seed production were described with a rectangular hyperbola model. The presence of wild radish in wheat reduced aboveground dry matter, leaf-area index (LAI), and grain yield of wheat, and the magnitude of this reduction was dependent on weed density. Increasing the density of wheat substantially reduced the adverse effects of wild radish on wheat. As crop density increased, wild radish dry matter, LAI, and seed production per unit area decreased. The maximum seed production of wild radish was achieved at its highest density (60 plants m−2), and was 43,300 and 61,200 seeds m−2 for the first and second year, respectively. The results indicated that higher densities of wheat were able to suppress seed production of this weed species. From a practical viewpoint, this study shows that increased wheat density in the range of 200 to 400 wheat plants m−2 can reduce wild radish seed production and also give some reduction in crop yield loss, and could be an important component of an integrated weed management program.