Latrunculin B is a potent toxin produced by the Red Sea sponge Latrunculia magnifica. Its mode of action is to depolymerize F-actin to form a G-actin-latrunculin complex. It has been shown to inhibit cell or tissue elongation in several plant systems, but apparently it does not inhibit tissue differentiation, i.e., miniature stature of tissues or plants result. A series of experiments were performed to determine if latrunculin B affects fiber elongation. One or 2 DPA ovules of Suregrow 125 were grown in 125mL flasks on 50 mL of Beasley-Ting medium with 10 ^-6 M each of IAA and GA. Treatments consisted of additions to the medium of latrunculin B dissolved in DMSO to give concentrations ranging from 0 to 5x10 ^-7 M. Ovules were cultured for two weeks, and each experiment of four replications per treatment was repeated. Fiber development was estimated visually, by staining with Toluidine Blue O and measuring spectrophotometrically the amount of strain recovered by destaining, and by measuring fiber length. Latrunculin B completely inhibited fiber elongation at 1x10 ^-7 M and caused at least a 10% reduction in fiber elongation when present as low as 1x10 ^-9 M. Ovules cultured for one day in 1x10 ^-7 M latrunculin B then transferred to toxin free medium for 14 days had fiber length inhibited by more than 50%. Latrunculin B clearly inhibited cotton fiber elongation at very low levels. The result implies that F-actin is essential for cotton fiber cell elongation.