The author had studied the age and rate of growth, the weight increase and the chemical compositions of eels living under the most typical conditions, such as: sea, littoral waters and inland lakes. The investigated material comprised 646 specimen of fish; out of this 499 represented the yellow eel (282 migrating from sea and river estuaries, 151 from Firth of Vistula River and 66 from inland lakes) and 96 represented the silver eels migrating to sea for spawning. 258 pieces were examined technologically and 158 were subjected to chemical analysis for content of proteins, fats, water and mineral compounds. The highest content of fat was found at the silver eels; lower content was noted at eels living in bays and still lower content, at eels living in lakes. The lowest rate of fat increase was observed at eels from inland lakes. Author concludes, that the content of fat in meat tissue is one of the characteristic conditions for silver eel, which stimulates it for spawning migrations. The existing literature in eel comprises strictly limited part of the problems such as, biological or technochemical, but does not comprise the problems in any complex form. Up to now, in Poland no detailed investigations have been carried out on the subject of eel. Aim of the present work was to determine the growth, changes in weight proportions and in chemical compositions of the eels living in Polish waters, with due consideration to the most typical conditions in economical aspect, such as: Baltic Sea-migrating and feeding (yellow eel) and silver eel. Mazurian Lakes-feeding eel (yellow). Firth of Vistula River-feeding eel. (yellow). The results of technochemical analysis permitted to define the usable and consumptive values of the fish.