Echinochloa glabrescensMunro ex Hook. f., a common weed in flooded fields, was highly competitive with rice (Oryza sativaL.) when its seedlings were transplanted with rice seedlings. Age of the transplantedE. glabrescensseedlings had little effect on its weight at harvest and rice grain yield at different infestation levels. Average rice yield reductions from transplanted E.glabrescensranged from 6% at the 5% infestation level to 73% at the 40% infestation level. Transplanting shock delayed vegetative growth of rice andE. glabrescensat early stages and influenced growth patterns until maturity. When all rice hills were infested, competition resulted in 11.9 and 12.8% reduction in height; 60.5 and 62.5% reduction in tiller number; 59.1 and 51.5% reduction in maximum leaf area index (LAI); 77 and 81% reduction in total dry weight; and 90 and 94% yield reduction of rice under transplanted and dibbled conditions, respectively. Panicle length and 100‐grain weight were not affected by competition.E. glabrescenswas less affected by competition than rice under both planting methods