Boll opening materials are often used in combination with defoliation materials to increase the percentage of the crop harvested during first picking or to possibly eliminate the need for a second picking. Common questions often asked by South Carolina cotton producers is how much time is required for bolls to open after boll opening materials are applied, how long is required until a certain percentage of bolls are opened after chemical treatment, and how many unopened bolls should be opened by boll openers in order for the chemical treatment to be profitable. Specific objectives of this study were: to determine the efficacy of PREP, FINISH, and COTTONQUIK on boll opening, leaf removal, lint yield, and fiber quality; to determine the time (based on heat units) required for various bolls located on the plant to open after boll openers are applied; and to assess changes in boll development and fiber properties at various fruiting positions in response to premature boll opening. Boll opening treatments consisted of : a) control - no boll opening materials, b) PREP applied at 1.33 pt/A (low rate), c) PREP applied at 2.67 pt/A (high rate), d) FINISH applied at 1 qt/A (low rate), e) FINISH applied at 2 qt/A (high rate), f) COTTONQUIK applied at 1 qt/A (low rate), and g) COTTONQUIK applied at 2 qt/A (high rate). Appropriate boll opening treatments were imposed at the 25 and 50% open boll stage by spraying individual plots with a ground sprayer. Although boll opening rates varied depending on the year and the crop condition, higher rates of boll opening materials usually caused more bolls to open earlier in the season compared to lower rates of the boll opening materials or the untreated check. No differences in lint yield were found among any of the boll opening treatments and the untreated check plots in 1999 or 2000 when applied at either the 25 or 50% open boll stage. Applying boll openers at both 25 and 50% open boll stage reduced boll size of the latest developing bolls. Early applications of boll openers significantly reduced micronaire of late-developing bolls.