RECENT observations on the brown root rot complex of tomatoes caused Last and Ebben1 to support the view of Termohlen2 that a fungus which produces a grey sterile mycelium in culture is responsible for the brown root rot symptoms. Preliminary studies on the occurrence of potato root eel worm and the grey sterile fungus (GSF) on the roots of tomato revealed that the development of the nematode is adversely influenced by the presence of the fungus. It was thought that the restriction of the reproduction of the nematode could be caused by some substance (or substances) produced by the fungus and which affect the hatch of potato root eelworm, or its subsequent migration to and development in the host plant roots. An investigation of the former is reported here.