Field studies were conducted in 1986 and 1987 to determine the critical period for weed control in white bean grown in Ontario. The treatments consisted of either allowing weeds to infest the crop for increasing durations after planting or maintaining plots weed free for increasing durations after planting. The beginning of the critical period was defined as the crop stage by which weed interference reduced yields by 3%. Similarly, the end of the critical period was defined as the crop stage to which the crop had to be weed free to prevent a 3% yield loss. The critical period of weed control occurred between the second-trifoliolate and first-flower stages of growth for all cultivars and years, with the exception of the cultivar ‘OAC Seaforth’ in 1986. The average number of pods per plant for both cultivars was reduced by increasing durations of weed interference after planting in both years. However, pod number of the cultivar OAC Seaforth was reduced at a greater rate in 1986 than ‘Ex Rico 23’. The beginning of the critical period corresponded with the beginning of a rapid increase in total weed biomass.