The effects of weed competition on cassava yield were assessed using two sets of treatments. In the first set, plots were kept weed free for specified periods and then weeds were allowed to grow. In the second set, weeds were allowed to grow initially for specified periods and were then eradicated. Effects were assessed from fresh weights of cassava roots, and weed species commonly found in the plots were identified. The work was done in the rain forest zone of western Nigeria. The predominant weed species were mainly annual broad-leaved species and competition from weeds at any period of growth after rooting reduced yield. The most damaging effect on yield was noted during early canopy formation and early tuberisation (third month after planting) and a less damaging effect from the fourth month until harvest. Where labour is a limiting factor in cassava production labour input should be applied during the third month after planting to control weed growth. Weeding during this period was as effective in ensuring a high yield of cassava roots than weeding throughout the entire period of growth.