Conidial traps (Ct) in Arthrobotrys oligospora are adhesive structures formed directly on germination of conidia, without an intermediate hyphal phase. A conidial trap-producing mutant was isolated by a procedure analogous to that used for microcycle conidiation. This Ct-strain produced strict conidial traps in abundance on conidia when still on upright conidiophores. The phenotypic stability of the Ct-strain lasted over more than 20 transfers on trap-inducing media. Growth on non-inducing media (5 transfers) and subsequent inoculation on trap-inducing medium again showed traps on conidia on upright conidiophores. The conidial traps attached to nematodes before invasion and digestion. At the subcellular level the Ct were characterized by the presence of many electron-dense microbodies. In addition, in the Ct-strain normal trap formation on the mycelium was considerably increased compared to the parent strain. This high trap-forming ability will make it a useful tool both in rhizosphere studies and in biochemical/molecular investigations.