Serum proteins of Tilapia mossambica. Tilapia nilotica and their hybrid (female T. Mossambica×male T. nilotica) were examined by using 7% polyacrylamide gelelectrophoresis.The T. mossambica serum was isolated into 12--14 protein bands, T. niloticaserum into 12--13 protein bands and the bybrid serum into 13--15 protein bands.Sexual differences in the electrophoretic patterns of serum proteins of the twospecies and their hybrids were compared. The electrophoretic patterns of bandingshowed two new bands in the serum of mature female T. mossombica. They were notobserved in the males. But in the females of T. mossambica and of T. nilotica theprotein bands were broder and darker in colour than those of the males. The eletropho-retic pattern of the hybrids was transient among several components.Various staining methods were used: (a) for total proteins with amide black dyetwo new protein bands appeared; (b) for glycoproteins with the periodic acid-Schiffreaction, the two new protein bands were not visible. However, other protein bandsshowed red colour;(c) for lipoprotein with sudan black B there were two markedprotein bands.For scanning electropherograms on serum of the fish and amins acid compositionof female specific serum protein,ultra-violet-spectro photometer was used. This femalespecific serum protein was characterized by relatively high contents of glycine, pheny-lalanine and amide NH. but the absence of histidine and tyrosine,