Currently in Roundup Ready cotton (RR), over-the-top applications of Roundup Ultra (RU) should be made prior to the fifth true-leaf stage. All subsequent applications must be post-directed with no herbicide contact to the leaves (Kerby and Voth, 1998; Roundup Ultra label, 1997). This application window limits the grower's ability to make over-the-top applications of RU, which may be necessary to control in-row weeds (Matthews et al., 1998). Applications of RU inconsistent with label directions have led to square and boll abscission resulting in yield loss (Ferreira et al., 1998; Kalaher et al., 1997). Moreover, significant yield reductions have also been observed with the standard RU program (Brown and Bednarz, 1998). Several other experiments have failed to show yield reductions with labeled and non-labeled RU applications (Blackley et al., 1999; Jones and Snipes, 1999; Matthews et al., 1999; Murdock, 1999), and only a small percentage of growers have reported concerns of boll abscission with RR technology (Heering et al., 1998). These contrasting observations warrant a better understanding of the response of RR cotton to RU applications; this may prevent potential yield losses due to possible boll abscission in RR cotton treated with RU.