Recent work in Baarn has shown that certain fluorescent Pseudomonads inhabiting the rhizospheres of potato and wheat can increase plant growth. The ability to increase growth is associated with the production of iron-chelating compounds (siderophores), which appear to starve as yet unknown deleterious rhizosphere micro-organisms of iron, thus preventing their growth and activity. The possible role of such bacteria and their siderophores in soil mycostasis was investigated. Two siderophore-producing fluorescent Pseudomonads produced zones of inhibited growth of the following six fungi on King’s medium B: Cochliobolus sativus, C. victoriae, Trichoderma hamalum, T. koningii, T. harzianum, and Botrytis cinerea. Fusarium solani f. sp. phaseoli was not inhibited. Spore germination within the zones showed little or no inhibition by isolate WCS 29 J, but isolate WCS 74 strongly inhibited germination of the Trichoderma spp. and B. cinerea. When Na-Fe-EDDHA (100 pM) was present in the medium, the zones of growth and germination inhibition either were not present (isolate WCS 29 J), or were reduced in size (isolate WCS 74), indicating the mycostatic potential of siderophores in vitro.