Soil samples were incubated on agar precolonised by different 'host' fungi for detection of presumptive mycoparasites. In a survey of 34 British soils, only Pythium oligandrum, Gliocladium roseum group, Trichoderma spp, and Papulaspora sp. were detected routinely, from 19, 34, 30 and 27 soils respectively; all soils contained more than one mycoparasite, 3 soils had two detectable types, 20 contained three types and 11 contained all four detectable types. The choice of host fungus strongly influenced the efficiency of detection of mycoparasites: Fusarium culmorum was best for P. oligandrum, Rhizoctonia solani for Trichoderma spp., and Botrytis cinerea for Papulaspora sp., but G. roseum (and G. atrum and G.fimbriatum) grew well from soil on colonies of Trichoderma aureoviride, R. solani or B. cinerea. The incidence of detection in replicate samples indicated that a range of host-colonised agar plates are needed to determine the spectrum of presumptive mycoparasites in a soil. Competition between these mycoparasites affected the success of detection because incorporation of metalaxyl into B. cinerea-colonised agar plates prevented growth by P. oligandrum and enhanced the growth of Papulaspora sp. from soil samples. Baiting of soils only partly increased the efficiencies of detection: for Trichoderma spp, when soil was baited with cellulose film precolonised by R. oryzae, and for Papulaspora sp. when the bait was cellulose film or mature, dried wheat leaves, precolonised by Humicola grisea. Agar precolonised by F. culmorum was used to detect apparent changes in soil populations of P. oligandrum, using a most probable number method based on serial dilution of soil samples with sand. The addition of dried, mature wheat flag leaves or fresh, green grass leaves to soil enhanced the detectable populations for at least 280 days, although the effect of grass leaves was found only after a second supplement at 150 days. A metalaxyl-tolerant population of P. oligandrum, added to soil as oospores, was still detectable after 240 days when soil samples were placed on F. culmorum-colonised agar containing metalaxyl.