Freshly collected sisal (Agave sisalana (Engelm.) Perrine) juice was fermented during ten days, spontaneously or by Saccharomyces cerevisae, either protected or unprotected from the environment. It was observed that the total sapogenins, hecogenin, tigogenin, two unidentified sapogenins, and total solids content were significantly dependent on the kind and duration of fermentation, the last characteristic being also dependent on the condition of fermentation. The higher total solids and sapogenins contents were obtained by spontaneous rather than by S. cerevisae fermentation, and, in both cases, under protection from the environment. There was, however, no significant correlation between total solids and sapogenins contents. The sapogenin forming reactions are oscillating reactions, in such a manner that the better yield of sapogenins are obtained at about the second day of fermentation.