The effect of density on individual and population fertility, and selected morphometric characters (height, number of: ramifications, generative branches, inflorescences and leaves) of Echinochloa o-us-gaili plants was studied in a glasshouse. A decrease in individual fertility was observed in all density variants examined, but it did not result in a decrease in population fertility per unit of area. In response to worse developmental conditions there appeared plants characterized by reduced height and simplified external conformation.