Mesorhizobium sp. F28 was isolated from a wastewater treatment system in a polyacrylonitrile (PAN) fibre factory for its ability to convert acrylonitrile by a nitrile hydratase (NHase)-amidase (AMase) system. This strain completely degraded acrylonitrile at concentrations as high as 18.4 mM, and exhibited 192 U mg(-1) dcw NHase activity for the conversion of acrylonitrile to acrylamide. The cobalt-containing NHase purified from the strain F28 was moderately thermostable, keeping full activity up to 45 C; it also had Optimum activity between pH 7.0 and 7.5. Acrylonitrile, propionitrile, isobutyronitrile, and benzonitrile were good Substrates for the NHase of Mesorhizobium sp. F28. The affinity of the NHase for acrylonitrile (K-m = 0.53 mM) was greater than that of bacteria mentioned by other researchers before. (c) 2008 Elsevier Ltd. All rights reserved.