The kinetics of survival and inoculum potential of Fusarium oxysporum f.sp. lini were studied in soil. Two types of inoculum were compared: microconidia freshly harvested from a laboratory-grown culture and microchlamydospores produced in sterilized soil. Introduced at the same inoculum densities into a natural soil, the two types of inoculum showed similar behavior; the inoculum densities changed little with time, at least during 100 days. However, the two types of inoculum did differ in disease potential. A higher percentage of microchlamydospores than microconidia germinated in the rhizosphere of flax seedlings, and the heterotrophic fluorescein diacetate hydrolysing activity of the microchlamydospores was 100 times higher than that of microconidia. Moreover, the microchlamydospores produced more disease on flax than the microconidia even at a much lower inoculum density.