Pea cultivars Onward, Alaska and Delwiche Commando, differential hosts for three physiologic races of Fusarium oxysporum f. pisi (cause of pea wilt), exert different effects on the soil microflora. Cultivar Onward, susceptible to race I, supports more fungi, bacteria and Actinomycetes near its root surface than do either Alaska or Delwiche Commando, which resist race I. Qualitative differences between rhizosphere populations of the three cultivars are most apparent in Fusaria and bacteria. The germination of spores of the three physiologic races of F. oxysporum f. pisi was affected by water extracts of the three rhizosphere soils in the same way as earlier with root exudates from the three cultivars. Spores of a race able to wilt a particular cultivar germinate well in rhizosphere soil extract from that cultivar. Germination is decreased, on the other hand, in extracts from the rhizosphere of a resistant cultivar. Spores of race 3, however, were not able to germinate well in rhizosphere extract of cultivar Alaska, which is susceptible to this race. Soils in which each of the three pea cultivars had grown were inoculated with spores of race I and planted with seedlings of the susceptible cultivar Onward. Plants became severely wilted in pots in which Onward had grown and in control pots which had not grown peas, but wilting was less and developed more slowly in pots which had previously been cropped with cultivars Alaska and Delwiche Commando (which resist race I). It is suggested that substances exuded by roots of cultivars resistant to race I, together with the effect of altering the soil microflora, prevent race I from germinating, thus lowering the amount of effective inoculum and delaying the onset of wilt.