A microbial process for the destruction of acetonitrile has been developed. Suitable microorganisms were enriched from activated sludge by cultivation with a high selection pressure when acetonitrile was the sole source of carbon and nitrogen. The process is based on cultivating the microorganisms as a biofilm on plastic carriers. These are utilised in a stirred-tank reactor with continuous feeding of 40-150 mM acetonitrile and an outlet for spent medium. The observed efficiency in the conversion process was from 53-100%, depending on the organic load. To improve the capacity and reliability of the process, a two-step process was developed with two stirred-tank reactors coupled in series. With such an arrangement, conversion efficiencies from 92-100% were achieved when the organic load was 2 g acetonitrile l(-1) d(-1) in the first reactor.