CARBON dioxide has been shown to influence morphogenesis in micro-organisms, higher plants and animals. Among the processes affected by carbon dioxide (or bicarbonate) concentration are sporangial differentiation in Blastocladiella emersonii1, mould-yeast dimorphism2,3, spherulation in Coccidioides immitis4, sporulation by bacteria5 and fungi6,7, root development by pea8, and sexual differentiation in Hydra9. This list can now be extended to include morphogenesis in Arthrobotrys conoides. A. conoides is a nematode-trapping fungus which captures prey in adhesive networks of hyphal loops. Usually, these organelles of capture are not formed spontaneously by the fungus but develop in response to the presence of prey. Morphogenesis is induced by a metabolic product of nematodes which has been partially purified10–12 and designated nemin13. The tests described here provide evidence that nemin-induced morphogenesis in A. conoides is dependent on carbon dioxide.