Many reports have regarded Caiman as a very important topic in the study of comparative anatomy, because this creature is a very attractive and significant animal, especially in oral science. It has been made clear by analyzing the structures of Caiman's tooth enamel and dentine that Caiman is in a transitional stage in evolution from amphibia to mammalia, morphologically. However, neither the structure nor the development of its periodontal tissue has been clarified so far. In this investigation, the fine structure of Caiman crocodilus tooth cementum was observed by using light and electron microscopy, in order to clarify the functions of human periodontal tissue, morphologically. The findings indicated that there are at least two layers in Caiman crocodilus cementum, which are named the inner layer of cementum (ILC) and the outer layer of cementum (OLC) in this paper. The ILC is much more mineralized than the OLC and the extrinsic fibers which mainly comprise the ground substance in both layers were revealed. Moreover, there are many cells in the cementum, which are confirmed cementocytes derived from mesenchyme and the epithelial rests of Malassez. Caiman crocodilus tooth cementum still has many primitive features, but has the fundamental structure to play a functional role as human tooth cementum, morphologically. © 2004, Japanese Association for Oral Biology. All rights reserved.