Root proliferation into soil zones rich in nutrients is an important mechanism ensuring effective exploitation of soil nutrients by plants. No studies have examined the effects of atmospheric CO2 enrichment on cotton (Gossypium hirsutum L.) root distribution patterns as affected by localizing P in decreasing volumes of soil. Cotton plants were grown in a Troup Sand using 17.2 liter containers placed in open top field chambers under ambient (360 ppm) or enriched (720 ppm) atmospheric CO2 conditions for 40 days. Equivalent amounts of P were added (150 mg P per kg of soil) to 100, 50, 25, 12.5, and 6.25 % of the soil volume; control containers with no added P were also included. Under extremely low P (controls), cotton was not responsive to atmospheric CO2 enrichment. In treatments with both fertilized and unfertilized soil volumes, root proliferation was greater in the unfertilized soil due to elevated CO2 conditions. Stimulation of root growth occurred in the P-fertilized soil fraction; the pattern of stimulation was similar under both levels of CO2. Under ambient CO2, cotton plants showed positive responses (shoot dry weight, and total root dry weight and length) to soil P when it was confined to relatively small proportions of the soil volume (12.5 and 6.25%). However, elevated CO2 grown plants tended to respond to P regardless of its distribution in the soil.