A fumigant, comprising cyanogen (C2N2) (as herein defined), provides a viable alternative to conventional fumigants such as methyl bromide, phosphine and carbonyl sulphide. Experiments have shown that cyanogen (C2N2) can be used as such a fumigant, in the control of a range of biota, including insects, mites, nematodes, fungi and their spores, viruses, spiders, bacteria, moulds and rodents. It is also suitable for use on a variety of commodities, including grain, seeds, meats, fruit, vegetables, timber, plants, cut flowers and soil. It is shown that an efficacious concentration of cyanogen (C2N2) as a fumigant leaves a very low residue and can be readily flushed from commodities. In particular, it has been observed that a mixture of cyanogen (C2N2) and carbon dioxide have a synergistic effect, when applied to various biota. Throughout the specification, it should be noted that the term 'cyanogen' is defined as the gas (at STP) cyanogen, C2N2, essentially free from hydrogen cyanide and other cyanogenic compounds (that is, compounds that give rise to hydrogen cyanide). Cyanogen, which has also been termed dicyan and oxalonitrile, as a fumigant, may be shown as (CN)2, C2N2 or NC-CN.