Fructose diphosphate aldolases have been isolated as crystalline proteins from rat liver, muscle and the Novikoff hepatoma. All three enzymes exhibit identical properties in sedimentation velocity and sedimentation equilibrium ultracentrifugation, corresponding to a molecular weight of 158,000. Dissociation in guanidinium chloride yields subunits of molecular weight approximately 40,000 in each case. Tumor and muscle aldolases exhibit identical electrophoretic properties on cellulose acetate or disc gel which are markedly different from those of liver aldolase. The tumor and muscle enzymes are 50 times more active with fructose-1,6-P2 than with fructose-1-P and treatment of these enzymes with carboxypeptidase A to remove the COOH-terminal tyrosine residues decreases the rate of cleavage of fructose-1,6-p2 so that it becomes equal to the rate of cleavage of fructose-1-P. Native rat liver aldolase, on the other hand, cleaves both substrates at equal rates and digestion with carboxypeptidase, while also liberating tyrosine from the COOH-terminus, does not affect the substrate specificity. The amino acid compositions of muscle and heppatoma aldolases were found to be identical and to differ significantly from that of the rat liver aldolase. Tryptic fingerprints of muscle and tumor aldolases are identical and differ significantly from that of liver aldolase. However, a number of peptides, including the active site peptide, are similar or identical in all three aldolases. The results provide support for the conclusion that the liver aldolase is replaced by muscle aldolase during the development of the Novikoff hepatoma.