A novel optical cotton yield monitor system has been under development at Mississippi State University since 1999, with laboratory and minimal field testing in that year. The system was field-tested more extensively in the 2000 harvest season. In 2001, the Mississippi cotton yield monitor was improved by adding temperature compensation and anti-stray-light features to the system. Five prototypes of the new version of the cotton yield monitor were fabricated. These prototypes were field- tested on three cotton pickers and two cotton strippers in five different locations across three States, Georgia, Texas, and Mississippi in 2001 season. More than 3,000 acres of cotton, with different varieties, and large yield variability, were harvested with the yield monitors from September to December of 2001. Results from the tests showed that the average absolute error of the system was 3.7% and 4.9% in two fields where system accuracy was evaluated load by load. All yield maps created with the data from the Mississippi cotton yield monitor realistically exhibited yield variations within fields, based on the expectations of experienced producers and consultants. The tests also indicated that the system was reliable, and easy to install, operate, and maintain.