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The aquatic fern Azolla Lam. is familiar in tropical agriculture as a nitrogen biofertilizer in rice paddies of Southeast Asia (IRRI, 1987). Its symbiosis with the nitrogen-fixing cyanobacterium Anabaena azollae Strasb. exploits a renewable resource. Expansion of Azolla useage to other rice-growing regions is a realistic possibility when this fern is also utilized as animal or poultry feed, or in a tripartite rice-fish-Azolla system. Agricultural application of Azolla is considered feasible in the neotropics. In Mexico, native Azolla species were found to grow rapidly, fix atmospheric nitrogen in significant amounts, and increase rice yield (Ferrera-Cerrato, 1980; Quintero Lizaola & FerreraCerrato, 1987; Quintero Lizaola et al., 1990). The difficulty of species identification, however, hinders the selection of desireable Azolla for field use. The morphological plasticity of New World taxa limits the utility of sporophytic characters for identification. Nonetheless, A. filiculoides (FI), A. caroliniana (CA), and A. mexicana (ME) have in the past been reported to be indigenous to Mexico (Svenson, 1944; Sota, 1976). Using moleculau criteria, CA, ME, and A. microphylla (MI) are now known to be very genetically similar. They may be regarded as a "species complex", while FI is genetically distinct (Zimmerman et al., 1991a, b). The range of MI, when treated as a separate species, is believed to exclude Mexico (Schofield and Colinvaux, 1969). This paper classifies and fingerprints Azolla collected in Mexico for agricultural purposes, using a diagnostic system of molecular markers. Growth results demonstrate the potential for this biofertilizer in rice agriculture of Mexico. METHODS