A copper-containing nitrite reductase (NiR) catalyzes NO2 1 reduction in denitrifying bacteria. The reaction mechanism was proposed based on the trimeric crystal structure. In this study, a small angle neutron scattering (SANS) experiment with NiR from Achromobacter cycloclastes (AcNiR) was performed, and the first solution structure was obtained. The obtained radius of gyration, the calculation of the scattering curve from the available pdb structure and ab initio modelling all indicate a trimeric structure of AcNiR in solution.