The influence of crop rotation and wheather conditions on the <i>Fusarium</i> population in soil and on <i>Fusarium</i> wilt of flax was examined in the Institute of Native Natural Fibres in 1968-1975. The experiments were carried out in various regions of flax planting in Poland during 8 years. Crop rotation had a significant influence on the process of biological soil disinfestation. <i>Fusarium</i> population in soil increased after planting flax, wheat, barley and sugar-beet, and decreased or did not change after rape, hemp, potato and leguminous plants. The crop rotation significantly influenced the composition of the <i>Fusarium</i> population as far as species are concerned. The highest wilt infection of flax was found in plantings after flax, sugarbeet and wheat and the weakest in plantings after rape. A six-year crop rotation was sufficient to eliminate the wilt pathogen from infested soil. But on flax-sick soil the process of biological disinfestation proceeded more slowly and a seven-year interval between flax crops was too short.