JANUS green, a bluish green basic dye of the morio-azo group, was observed to react in an interesting and useful manner with both nematode-trapping fungi and their prey. The dye is used at a concentration of 0.01 per cent in 0.2 M sodium acetate–acetic acid buffer at pH 4.6, and added drop by drop to the test material. Excellent results were obtained when the staining solution was applied directly to surface cultures of Arthrobotrys conoides, A. dactyloides and Dactylella ellipsospora, on maize-meal extract agar and on ‘Cellophane’1. These fungi capture nematodes by means of adhesive hyphal loops, constricting rings, and adhesive knobs, respectively. The nematode used was Panagrellus redivivus.