Esterases from different body organs of two colour morphs of the freshwater eel, Anguilla anguilla were extracted and analyzed using starch gel electrophoresis, five substrates and four inhibitors. Eight esterase bands were recognized in both colour morphs. Bands were distributed in two main zones. They exhibited different tissue distribution, and different substrate specificity. As the results of inhibition studies, esterases were classified as carboxylesterases and acetylesterases. Depending on substrate specificity, inhibition characteristics and relative mobility, esterase bands
could be traced to three genetic loci. One locus codes for bands of zone I. The other two loci code for bands of zone II. No significant differences could be found between the two colour morphs.