Growth hormone is an essential polypeptide required for normal growth and development of vertebrates. The pejerrey fish, Odontesthes bonariensis, is a South American atherinid freshwater fish considered as a promising species for aquaculture. Although growth hormone has been characterized in a number of fish, there are no published data on the structure of this hormone in atherinids, except that of a related species Odontesthes argentinensis. In this paper, the molecular cloning, expression and immunological characterization of pejerrey growth hormone (pjGH) is described. The predicted amino acid sequence of pjGH cDNA consisted of 204 amino acid residues with an estimated molecular mass of 23 kDa. Amino acid sequence was highly conserved among the two Atheriniformes where the growth hormone sequences are known (99% aa identity), highly to moderately conserve (75-92% aa identity) when compared to the other members of Acantopterigii superorder and clearly less conserved (49-66% identity) when compared to Salmoniformes (Protacanthopterygii), Cypriniformes and Siluriformes (Ostariophysi). A phylogenetic tree depicting the relationship of various teleost GH nucleotide sequences was inferred. Pejerrey GH was produced using recombinant DNA technology in a bacterial system, representing the first time an atherinid growth hormone protein was expressed as a recombinant protein in Escherichia coli. A specific antiserum of this hormone was raised in rabbits and its specificity tested by using Western blot and immunocytochemistry. The distribution of pjGH mRNA was also studied by RT-PCR and Southern blot analysis. The transcript was detected not only in the pituitary gland but also in the testis.