Abstract: Laboratory study was conducted to determine the nitrification of ammonium sulphate, via nitrite, to nitrate by Fusarium species grown under oligotrophic conditions. The addition of ammonium sulphate led to a marked increase in the concentration of nitrate, with a slight increase in nitrite, in most species. The maximum amount of nitrate was recorded in a medium inoculated with fungi of Fusarium solani (3.4 μg/ml) and Fusarium oxysporum (2.3 μg/ml). These species exhibited the largest content of protein. Fusarium ciliatum (0.7 μg/ml) reached the lowest amount of protein production when grown under oligotrophic conditions.