ABSTRACT Carbamoyl phosphate synthetase (CPS) is one of the large and ancient gene with tandem duplication, universal distribution, constant function and highly conserved over great phylogenetic distance. The CPS catalyses the formation of carbamoyl phosphate from CO2, ATP and ammonia or glutamine for pyrimidine biosynthesis, arginine biosynthesis or urea cycle. It has been suggested that all three forms of CPS have evolved from a progenote, kinase gene, which had undergone an ancient duplication. In an attempt to study CPS isoenzyme genes in H.fossilis this is first partial unique sequence of CPS gene of 215 base pairs from H. fossilis. A comprehensive phylogenetic analysis considering the CPS sequences from representatives of all three domains of classification archea, bacteria and eukarya indicates unique CPSI in air-breathing teleost, H. fossilis. Keywords: Carbamoyl Phosphate Synthetase, Heteropneustes fossilis, evolution, phylogeny, Ureotely