Cotton (Gossypium hirsutum L. cv DPL 5690) fibers develop from single cells in the epidermis of ovules. During maturation, the fiber cell deposits a thick secondary cell wall composed almost entirely of cellulose. Cellulose accounts for >90% of the dry weight of the mature fibers, and therefore, its biosynthesis constitutes the major metabolic function of these cells. Herbicides which inhibit cellulose synthesis, i.e., isoxaben or dichlobenil, were employed in ovule culture to elucidate the physiology of fiber development. Fibers were grown in media containing 1X10^-5 to 1X10^-9 M isoxaben or dichlobenil. After a one week incubation, growth of the fibers as a function of herbicide concentration of fiber growth as a function of herbicide concentration was plotted. Fiber growth was completely inhibited by incubation in 1X10^-5 to 1X10^-6 M isoxaben or in 1X10^-5 to 5X10^-7 M dichlobenil. Light microscopy of these treated ovules showed that the fibers topped elongating and became irregular in shape, usually swollen between their base and tip. Similar to fiber, cells in the see coat also became swollen, often enlarging 5-10 times the size of the cells in the control treatments. This cellular enlarging could also explain the ovules developing into callus type growth, especially in 1X10^-6 M isoxaben of 5X10^-7 M isoxaben or 1X10^-7 M dichlobenil, Many fibers from these ovules appeared to elongate normally, however, others were multicellular. Fibers grown in 1X10^-8 to 1X10^-9 M isoxaben or dichlobenil appeared normal at the light microscopy level. These data indicate that fiber growth is effectively disrupted by two herbicides known to inhibit cellulose biosynthesis. Electron microscopic and biochemical characterization of the affected precess may indicated more about the physiological events of fiber elongation and growth.