The cotton (Gossypium hirsutum L.) plant responds to a doubling of atmospheric C O 2 with almost doubled yield. Gas exchange of leaves was monitored to discover the photosynthetic basis of this large response. Plants were grown in the field in open-top chambers with ambient (nominally 350 #1/1) or enriched (nominally either 500 or 650 #1/1) concentrations of atmospheric CO2. During most of the season, in fully-irrigated plants the relationship between assimilation (A) and intercellular CO2 concentration (c i) was almost linear over an extremely wide range of c i . CO2 enrichment did not alter this relationship or diminish photosynthetic capacity (despite accumulation of starch to very high levels) until very late in the season, when temperature was somewhat lower than at midseason. Stomatal conductance at midseason was very high and insensitive to CO2, leading to estimates of c i above 85% of atmospheric CO2 concentration in both ambient and enriched chambers. Water stress caused A to show a saturation response with respect to ci, and it increased stomatal closure in response to CO 2 enrichment. In fully-irrigated plants CO 2 enrichment to 650 #1/1 increased A more than 70%, but in water-stressed plants enrichment increased A only about 52%. The non-saturating response of A to ci, the failure of CO2 enrichment to decrease photosynthetic capacity for most of the season, and the ability of the leaves to maintain very high ci, form in part the basis for the very large response to CO2 enrichment.