The influence of potassium cyanide (KCN), dissolved O-2 concentration and medium composition on alternative respiration (AR) of Acremonium chrysogenum were investigated. The respiration of the fungus was only partially inhibited by KCN, but completely inhibited by the combination of KCN with salicylhydroxamic acid. It has been proved by in-situ measurements of the NADH-dependent fluorescence that the AR is active at low dissolved O-2 concentrations. The influence of the medium composition and the age of the fungus on the specific oxygen uptake rate is considered.