To determine the effects of intra-row competition on plant growth and development, two cultivars of cotton (Gossypium hirsutum L.), 'DES 56' and 'DPL 61' were grown in 1981 and at five population densities, ranging from approximately 19,700 to 199,000 plants/ha. In both years, significantly taller plants were obtained during squaring and early flowering in plots with high population densities. Continued increases in plant height during the flowering period were, however, greatest in the low-population plots. Similarly, early flowering intensity was positively related and late flowering intensity was inversely related to population density. In 1981, both firstharvest and second-harvest seedcotton yields were significantly influenced by population density: first-harvest yields increased in a curvilinear fashion with increasing populations, through the medium density; second-harvest yields decreased nearly linearly with increasing populations. Total seedcotton harvested was greatest at the medium population, although yields were not significantly different among the population densities studied. In 1982, when first-harvest yield was measured when the crop was 75% open, neither first-harvest, second-harvest, nor total seedcotton yields were influenced by production population density. In both years early-season growth was increased by high plant populations, but these increases came at the expense of late-season growth. Even though the cultivars studied differ in maturity characteristics, there were no significant interactions between cultivars and population density for the growth characteristics measured. These data indicate that high plant populations require more than can be obtained from the production environment in order to sustain late-season growth and development. Consequently, when environmental conditions might allow productivity during the middle and latter stages of the season, the more determinate plants grown at high levels of competition are unable to take advantage of these conditions, and production may be reduced.