Aims: To investigate the effects of NaNO2 on the microaerophilic flagellated protozoan, Tritrichomonas foetus KV1, an economically important cattle parasite that inhabits the vagina and can spread rapidly through herds of animals by sexual transmission and leads to abortion of foetal calves. Methods and Results: Growth of the parasite was inhibited by 50% in the presence of 4 mm NaNO2 ; immediate killing occurred at 10 mm. Mass spectrometric monitoring of gases showed that H-2 and CO2 evolution were inhibited by NaNO2 , and electron paramagnetic resonance spectrometry revealed a signal similar to that of a thiolate-iron-NO complex. Growth with sublethal concentrations of NaNO2 yielded organisms that produced ethanol rather than H-2 . Conclusions: NaNO2 probably inactivates FeS protein(s) of hydrogenosomes so as to inhibit the conversion of pyruvate (derived from maltose in the growth medium) to H-2 and acetate. Significance and Impact of the Study: The use of NaNO2 as a topical antitrichomonal agent in veterinary practice is a possibility. At present, slaughter of infected animals is the favoured method of control.