Biology of Colossoiiza inacropoiiztinz (Teleostei: Serrasalmidae) in the Mamore basin (Bolivian Amazonia). The biology of Colossonza nzacroponzun? is studied in the Trinidad region (Mamore Basin, Bolivia) and tlie results are compared with those from C. inacroponzum stocks are already in regression in central Amazo he two main water types, white waters originally from the Andes and black waters formed in 'pampas'. It is outstandingly resistant to hypoxia through 'the development of lower lip extensions often obsehed in numerous adults. During high waters, C. inacioponzunz are scattered mto the flooded areas; they gather together m the lakes when the wafer recedes and finally reach the Mamore river. At low water, they migrate upstream for reproduction which takes place in the last quarter of the year near the Securé riverbanks. In the breeding season, males remain at tlie spawning places while, after spawning, females move to recently flooded areas. The length at sexual maturity is 62 cm for females and 60 cm for males. There is a clear condition cycle composed of rapid fattening during high waters followed by a long thinning down beginning at fall and lasting till the middle of next flood. These results on feeding agree with those obtained in central Amazonia: intense feeding period on fruits and seeds falling from the trees during high waters followed by a long fast, total for adults, partial for youngs. This cycle is registered on otolith (asteriscus) sections by alternating broad opaque zones and hyaline rings well stained by toluidin blue. These marks were useful to age most fishes with a good accuracy and to study the growth. The growth is fast and similar in both sexes until sexual maturity, which occurs between 7 and 10 years. Adult male growth is slightly slower than that of female; the difference between the two average maximal lengths is only 2 cm (74 cm for females, 72 cm for males). CoZossoi?za nzacropomziin can live a very long time, at least 40 years (maximum, 65 years). Growth curve shows a pattern characteristic of inexploited stocks. Despite scientific and economic interest, our knowledge of C. iizacropoinuin is still poor and a great effort is necessary before the human impact, already important in some regions, considerably reduces the stocks and complicates their study.