Competition effects of four of the predominant grass weed species in Ethiopia (Avena abyssinica Hoechst, Lolium temulentum L., Snowdenia polystachya Fresen (Pilg), and Phalaris paradoxa L.) on the morphological characters, grain yield, and yield components of bread wheat (Triticum aestivum L.) were studied in western Shewa Zone of Ethiopia. The four grass species varied significantly in their effects on wheat plant height, tillering, leaf area index (LAI), number of fertile spikes m^-2, grains per spike, spike length, thousand grain weight, straw, biomass, harvest index and grain yield. A. abyssinica and S. polystachya were the most competitive, reducing wheat morphological characters and yield components to a greater extent than L. temulentum and P. paradoxa. The reduction in wheat grain yield was linearly proportional to the square root of weed seedling density. Grass species by seedling density interaction effects were significant for most of the crop and weed characters measured, indicating a differential rate response for individual species. The reduction in wheat grain yield at the maximum weed density of 320 seedlings m^-2 ranged from 48 to 86% across the four grass species studied. The wheat yield components most affected by weed competition were number of fertile spikes m^2 and number of seeds/spike. Weed vegetative and reproductive characters (i.e., number of tillers, LAI, number of panicles, and plant height) varied markedly among species and in direct proportion with weed seedling density. Plant height and LAI appeared to be the factors most closely associated with weed competitive ability with bread wheat.399-409