Commercial samples of mabí, mabí bark and refined sugar were obtained from various sources and examined for the types of microorganisms in them. The principal microorganisms found in commercial mabí were: Enterobacter aerogenes (1); Enterobacter cloacae (2); Citrobacter intermedius (3); and the yeast Saccharomyces cerevisiae (4). Escherichia coli (5) was isolated from one sample only. Mabí bark contained 2 and 3, while sugar samples contained 1, 3, and 4. An Asperigillus species and a Penicillium species were found in the sugar samples. Mabí prepared in the laboratory with pure cultures of (1 + 2 +3 +4), or (1 + 4) tasted like commercial mabí, according to 78% of the members of a taste panel. It is concluded that mabí fermentation is a natural fermentation carried out mainly by bacteria 1 and 2, the yeast Saccharomyces cerevisiae, and possibly by Citrobacter intermedius. E. coli does not seem to be involved, unless unsanitary conditions prevail during the preparation of the drink.