Abstract: Genetic relationships of cercopithecid species in Thailand are determined based on sequence analysis of the internal transcribed spacer (ITS) regions of ribosomal DNA (rDNA). Aligned sequences of the complete ITS regions obtained from the six taxa and an outgroup, Pan paniscus resulted in 634 characters. Within the Cercopithecidae, ITS sequences are highly conserved with 97.48% homology. The resulting comparisons of ITS sequences were used for analysis of genetic distance and a dendrogram was constructed based on the Neighbor Joining Method. From the dendrogram, Pan paniscus is solely separated. The Colobinae species, Trachypithecus cristatus, T. phayrei, T. obscurus, were placed inside the Cercopithecinae species, Macca arctoides, M. mulatta, M. nemestrina. The dendrogram of these species does not completely match the former classification based on morphological characters. Genetic distances are very low (0.002 to 0.013) among Thai cercopithecid species. The results suggest that the ITS regions of rDNA is highly conserved in Thai cercopithecids.