Interference between itchgrass and soybean was evaluated over two years using area-of-influence procedures. Soybean seed weight within 20 cm of the weed was reduced 15 to 21%. In the first year, seed weight reduction of 9% was detected at a distance of 40 to 60 cm from the weed. Weight reductions were attributed to decreases in seed number of 12 to 22% within 40 cm of the weed compared with the 140 to 160 cm control area. Itchgrass interference increased soybean height within 40 cm of the weed, but soybean canopy width was generally unaffected. Soybean interference did not affect height of itchgrass plants, but did reduce weed stem numbers 89 to 94% when compared with weeds growing alone. With interference of 8 wk or less, both itchgrass green and dry weights were similar when weeds were growing alone or within the soybean row. Green and dry weights were reduced approximately 80% when itchgrass was in competition with soybean for 10 wk.