Technical Abstract: Vegetative diversification can enhance natural enemy populations and suppress pest damage in various crops. Weedy and weed-free cotton plots were studied to examine the effects of weediness on arthropod prey and predator populations, boll weevil mortality and injury to cotton squares, and on cotton plant growth and yield during 2000 and 2001. Presence of weeds was associated with significantly higher populations of 81% of the prey arthropod groups and 69% of the predator arthropod groups that were counted, especially late in the season when weed biomass was highest. However, weed-free cotton harbored more cotton aphids and whiteflies than weedy cotton on some of the sampling dates. Diversity (Shannon's index) was significantly greater in arthropod samples collected from weed foliage and weedy cotton plants than from weed-free cotton plants, but diversity was not different between treatments for ground-associated arthropods. Boll weevil oviposition injury to cotton squares was unaffected by weeds because the higher weed-associated predator populations mainly occurred after most squares had become less vulnerable bolls, and because the natural enemies encountered in this study are not effective against boll weevil populations. Weed competition was found to be responsible for the lower lint yields in the weedy cotton.