In vitro growth studies showed thatArthrobotrys oligosporaFresenius,A.conoidesDrechsler,A.musiformisDrechsler, andA.robustaDuddington are characterized by a fast, inherent growth rate relative toA.dactyloidesDrechsler,Dactylella gephyropagaDrechsler, andDactylaria brochopagaDrechsler, which grew slowly on all media tested.Arthrobotrys superbaCorda,Dactylella ellipsosporaGrove, andD.asthenopagaDrechsler were found to be intermediate in this respect. There was little correlation between the maximum rate of increase in colony diameter of the 10 species of nematophagous Hyphomycetes and their ecological requirements for such growth. The fast-growing species appeared to prefer media with a pH range of 5.0–6.0 and the slow-growing fungi a pH of about 4.0. No consistent similarity in environmental requirements was found among species of fungi belonging to the same genus or between those possessing the same kind of nematode-trapping organ. Lima bean agar of pH 5.0 was a satisfactory culture medium and incubation at 25 °C in continuous or intermittent light resulted in larger colonies than incubation in darkness.