The results showed that, in general, for all the fungus-nematode combinations studied, an inital population of A. avenae at approximately 1710-8550 nematodes gave the best control of flax damping-off in 133cm^2 of R. solani sick soil. Both higher and lower populations gave poorer control of the disease. Similar resuslts were obtained from Rhizoctonia darning-off of Jute. A. avenae removed cell contents of hyphal cell of R. solani in a feeding cycle of about 15 sec. duration. The nematodes probed with its lips before feeding, turning its head from side to side until its lips touched a host cell. The nematode oriented its stylet against the cell wall at a right angle to the cell surface. The rate of population increae of A. avenae was great at moderate temperature (30℃), and was least at both very low (10℃) or very high temperature (40℃). Soil moisture was also an important environmental factor affecting population growth. In general, the number of nematodes surviving in soil decreased with the increase of soil moisture. The optimum growth soil moisture was 20%MHC.