Nitric oxide ( NO) is a highly reactive, labile gas produced by the enzymatic conversion of L - arginine by nitric oxide synthases NOS. In mammals, NO mediates multiple physiological processes from cardiovascular control to neural transmission. Three NOS isoforms neuronal, inducible and endothelial have been cloned, sequenced and characterized from several mammalian species. nNOS isoforms were characterized from adult grouper ( Epinephelus coioides ) by using RT- PCR with degenerate oligonucleotide primers designed against a portion of the mammalian NOS gene that codes for the calmodulin - binding region, this region was chosen because it is highly conserved among NOS sequences to date and is a functionally important region of NOS proteins. A partial gene sequence of 377 bp corresponding to mammalian nNOS is obtained. This sequence showed 82% - 83%, 85% - 93% homogeneity with that of mammalian and the other f ish respectively. The deduced amino acid of E . coioides show s high identity with that of mammalian ( 92% - 93%) , and the other fish ( 93%- 99% ) . Phylogenetic analysis of these sequences conf irms the conserved nature of NOS, particularly of the calmodulin- binding domains.