To evaluate the sperm motility stimulating activity of a sulfono glycolipid (S-ACT-l) isolated from Gelidiella acerosa, a Sri Lankan marine red algae. Methods: S-ACT-l, a white amorphous powder was separated from more polar fractions of the hexane soluble of 1:1 CH2Cl2/MeOH extract and subjected to 1H, 1 3C NMR and IR Spectroscopy after reverse phase HPLC for identification. Effects of S-ACT-1 on human sperm motility was assessed in vitro at 10,100 and 1000μg/Ml concentrations at 37℃ for 0, 5, 15, 30 and 60 min. Results: S-ACT-1 was identified as a glycolipid sulfate. The lower dose increased the sperm motility slightly, whilst the medium dose significantly increased the motility ( P ＜ 0.05) from 5 min of incubation reaching a peak at 15 min and the stimulant effect was sustained throughout the experimental period. Furthermore, the medium dose rendered 80% of the immotile viable sperm motile.In contrast, the highest dose impaired the sperm motility. The sperm stimulating activity of S-ACT-1 was dose-depen dent and had a bell-shaped dose response curve for all the 5 incubation periods. Conclusion: S-ACT-1 of Gelidiella acerosa is a Sulfono glycolipid. S-ACT-1 has a potent sperm motility stimulating activity in vitro and has the potential to be developed into a sperm stimulant.