Using 12 cultivars of barley, density response was investigated at five levels of spacing, i. e. 2x2cm2, 4x4cm2, 8x8cm2, 16x16cm2 and 32x32cm2 per plant. The characters examined were culm number, ear number, seed yield and plant weight. Decrease in plant density caused proportional increase in vegetative growth, that is culm number and ear number, but seed yield tended to reach a plateau when the spacing surpassed a certain spacing level though the critical density was different among genotypes. Density response in vegetative growth and that in reproductive capacity were not correlated. Competitive ability of the same 12 cultivars was tested by a common test-variety. Comparison between competitive ability and density response showed that they were not found to be highly correlated, suggesting that they are different rather than the same traits. Nine factors which are supposed to influence the growth of individual plants in a population are listed together with some considerations.