Field experiments were conducted at Morro Agudo county, SP, Brazil, to determine the critical period of weed control in peanut. A different cultivar was planted for each experiment (runner cultivars: 'Caiapó ' and Runner 'Tégua', and erect cultivars: 'ST-Tatu', 'IAC-1075' and 'IAC-22'). Treatments consisted of increasing initial periods of weed maintenance or weed control, in a randomized block design with four replications. The predominant weed species were Brachiaria plantaginea (Link) Hitchc. (alexander grass), Digitaria horizontalis Willd. (crabgrass) and Bidens pilosa L. (hairy beggarticks). In the absence of weeds the most productive cultivar was 'Caiapó ', followed by Runner 'Tégua', 'IAC-22', 'ST-Tatu' and 'IAC-1075'. Weed interference resulted in maximum yield losses between 74 and 92%. Assuming a level of 5% in peanut yield loss, the critical time of weed removal was 7 to 16 days after planting (DAP) and the critical weed-free period was 26 to 65 DAP. Thus, to ensure the high yield for all cultivars the critical period for weed control should begin at 7 DAP and continue until at least 65 DAP.