DSM, a large chemical company in the Netherlands, operates a plant for the biological treatment of the wastewater of about 35 production plants (organic and inorganic chemicals). A special feature of this biotreatment plant is complete nitrification of Kjeldahl-nitrogen, in combination with denitrification. Experiences with this installation are reported. A serious odour problem has arisen due to the presence of traces of a tellurium-containing catalyst from one of the plants. An organic tellurium compound with a very low odour threshold is produced in the wastewater treatment plant, mainly under anoxic, dentrifying conditions. Experimental results on the origin of the problem are reported, and a solution is given.