Pieces of propylene oxide fumigated banana leaves, and detached roots ofMusa balbisiana, were used as substrates to study the colonizing ability ofF. oxysporumf.cubenseclone C in competition with the natural flora of different soils. Populations of clone C ranging from about 5000 to 100,000 spores per gram of soil colonized in 3 days a variable but low percentage of baits, usually being recovered from less than 20% of the leaf and root fragments sampled. Colonization of leaf baits also occurred in submerged soils containing 60,000 or more spores per gram of clone C, but was reduced one-half or more in comparison with non-submerged soil. In Honduras banana soil,F. solani, Rhizoctoniaspp., and sometimesRhizopusspp. were dominant colonizers. SaprophyticF. oxysporumwas also recovered from some of the baits along with clone C. Clone C was recovered from a small percentage of grass roots colonized in situ. In diseased banana plants, clone C was a predominant colonizer of portions of about 20% of the decaying main roots. Colonization presumably occurred outward into the decaying cortex from a parasitically established base in the infected stele.In banana soilsF. oxysporumf.cubense, even at populations of 100,000 per gram of soil, was a weak competitor in comparison withF. solaniandRhizoctoniaspp. at natural populations of less than 2000 per gram of soil. Natural populations ofF. solaniandF. roseumdominatedF. oxysporumin general in saprophytic competition for various substrates. Nevertheless, the results showed thatF. oxysporumf.cubensemust at times take advantage of favorable micro-environments to successfully invade a small percentage of colonizable sites on roots and other substrates. In these instances, the banana wilt pathogen has probably escaped competition with more vigorousFusariumand other saprophytes because of random spore distribution in soil.