Hydrazine is rarely found as an intermediate in microbial nitrogen conversions. In this study the conversion of hydrazine by the anaerobic ammonium oxidation (Anammox) culture, in which hydrazine has been proposed as an intermediate, was investigated. This study demonstrated the biological nature of hydrazine conversion by the Anammox culture. In batch cultures with hydrazine it was observed that 3 mol N2H4 was disproportionated to 4 mol NH4+ and 1 mol N2. Hydrazine with nitrite as an electron acceptor showed a conversion of 3 mol N2H4 and 4 mol NO2- to 5 mol N2, with a specific activity of 5.5 nmol min-1 (mg volatile suspended solids)-1. Addition of hydrazine to a biofilm reactor for 80 days showed that it was not possible to grow Anammox with hydrazine.