Gender identification of fish species is carried out mainly by examining external morphological characteristics, which in general, it is very complex and not always a reliable approach. Electrophoresis of plasma proteins can be used as an alternative and useful molecular tool for a more precise sex determination. The presence of female specific proteins in the plasma is a starting point for the application of this technique. In this study, reducing discontinuous sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was applied to analyze plasma proteins of male and female koi carp (