L-lactate dehydrogenase (LDH) cDNAs encoding for LDH-A4 (muscle) and LDH-B4 (heart) isozymes from caiman (Caiman crocodilus apaporiensis) belonging to the order Crocodilia were sequenced. The phylogenetic relationships of the newly determined cDNA and their deduced protein sequences, as well as the previously published sequences of vertebrate LDH isozymes were analyzed by various phylogenetic tree construction methods. These results indicated that Chelonia is indeed more closely related to Crocodilia. The divergent times between caiman and alligator, Chelonia and Crocodilia, were estimated to be approximately 36, 177 million years, respectively. £`-crystallin/Lactate dehydrogenase B cDNA from caiman (Caiman crocodilus apaporiensis), Pekin duck (Anas platyrhynchos), Muscovy duck (Cairina moschata) and Greylag goose (Anser anser) eye lens were sequenced. Accorcding to cDNA sequences, duck lens £`¡Vcrystallin and heart LDH-B are the products of the same gene. In amino acid sequences, two residues Asn-114 and Phe-118 are well conserved in£`-crystallin/ LDH-B among caiman, Muscovy duck and Greylag goose except in Pekin duck which are replaced by glycine residues. The lens protein composition, LDH activity and£`-crystallin/ LDH B4 protein structure of caiman and three avians were analyzed and compared. The results show no significant differences in conformational or enzymatic properties between Pekin duck £`-crystallin and caiman, Muscovy duck and Greylag goose £`-crystallin. The unique replacement of both Asn-114 and Phe-118 by Gly residues in Pekin duck £`-crystallin amino acid sequence might therefore be due to the selective pressure during the recruitment processes of active enzyme into avian lens£`-crystallins.