For the first time silicon has been discovered in closed boll cotton fibers. This discovery initiated an investigation into why, how much, and at what stages of growth was silicon present. Concentrations of silicon at various stages of fiber growth showed the same trend in all varieties for all years studied. The percent silicon decreases with age as the fiber develops. However, the concentration of silicon calculated as milligrams of silicon per mass of fiber per seed showed a definite peak at three weeks postanthesis. This is the time that fiber elongation slows down and secondary wall cellulose is initiated.