Injury symptoms that may be observed with pyrithiobac (the active ingredient in Staple®) are leaf chlorosis and plant stunting. While leaf chlorosismay be transient in nature, it could result in reduced carbon exchange rates, which could result in reduced crop growth and lint yield. This study was conducted to determine if pyrithiobac alters carbon exchange rates in cotton (Gossypium hirsutum L.) and, if so, to what magnitude and duration. Three-week-old cotton plants "SureGrow 125" were placed inside transparent chambers after foliar application of tap water (control) or one of three pyrithiobac treatments. The transparent chambers were then placed inside growth chambers and crop carbon exchange rates were measured every 20 minutes for 14 days. Daily averages of net photosynthesis, dark respiration, daily carbon gain, gross photosynthesis, and carbon use efficiency were determined from the gas exchange data. Significant reductions in net photosynthesis, daily carbon gain, carbon use efficiency, and cumulative carbon gain were consistently observed with the highest rate of pyrithiobac. However, these were transient reductions and by 12 days after treatment established carbon exchange rates were not different from the untreated plants. The 10 days of reduced carbon exchange rates from the highest rate of pyrithiobac would represent a 0.65% reduction in aboveground dry mass at cutout, which is considered minimal. However, the severity and duration of post-emergence, over-the-top applications of pyrithiobac under adverse field conditions could be extended.