Nitrification with pure O-2 of fisheries wastewaters high in N-NH4+ (0.2-1 g/l) and in organic matter (1-2 gCOD/l) gave specific growth rates for the bacteria degrading organic matter of 0.24 h(-1) and for nitrifying bacteria of 0.15 h(-1). Average organic matter abatement was 75% whereas N-NH4+ (0.7 g/l as initial concentration) oxidation to nitrite and nitrate was 20%.