The primary structure of rabbit liver fructose-l,6-bisphosphatase was determined by peptide analysis of digests with different proteases. The results establish the primary structure, complete data bank entries, and show that this enzyme variant is indeed homologous with other liver fructose-l,6-bisphosphatases. Residue differences with the enzymes from other mammals are 9-15%, with those from plants and yeasts about 50%, and with those from characterized prokaryotes up to 70%, showing an enzyme variability intermediate between those of 'variable' and 'constant' oligomeric dehydrogenases. Structural relationships, conformations and catalytic mechanisms are consistent within the family of fructose-l,6-bisphosphatases, and the rabbit protein is a typical rather than an aberrant form of the enzyme.