Bloodstream forms of Trypanosoma brucei lack cytochromes and are, therefore, insensitive to cyanide. Azide is a toxic anion that bears chemical and biological properties in common with cyanide and may act in a similar way by inhibition of cytochrome c oxidase. It was, therefore, surprising to find that bloodstream forms of T. brucei are sensitive to azide; growth is reduced by 50% with 0.1 mM azide. So far, the only enzyme known in bloodstream forms of T. brucei to be sensitive to azide is the iron-containing superoxide dismutase. However, because the activity of the superoxide dismutase was not affected in parasites incubated for 16 hr with 0.5 mM azide (a concentration at which no cell proliferates), the toxic action of azide cannot be due to inhibition of this enzyme. These results indicate that the general toxicity of azide is different from that of cyanide.