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Inoculation of soil by Trichoderma fungus is known as an effective method for biological control of soil borne plant pathogenic fungi. The population dynamics and longevity of Trichoderma propagules can be highly influenced by soil conditions. The effect of five factors including humidity of 30, 40, 50, 60, and 70%; temperature of 10, 15, 20, 25 and 30°C; pH of 3.8, 4.9, 6.7 and 8.1; organic matter of 0.8, 1.3, 1.8, 2.5 and 3.2%; biological activity of 0, 25, 50, 75 and 100% (percent non-sterile in sterile soil) on the population dynamics and survival rate of Trichoderma harzianum H32 in soil were studied in the course of five experiments. The characteristics of the original soil were: loamy clay sand (loam 58%, clay 26%, sand 16%), pH 8.1, EC—5 dS/m and organic matter 0.8 1%. Initial population of fungus was adjusted to
spore/g dry soil by adding a suspension of spore (106 spore/ml) to the soil samples. Each experiment was performed by using a completely randomized design with five treatments and four replicates in controlled conditions. Sampling was accomplished every 10 days for 90 days. Population of propagules (cfu) was determined using serial dilution method on semi selective medium of Papavizas & Lumsden. The data analyzed, using SAS softwear, demonstrated that the propagule population begins to decrease starting from day 10 and continues decreasing with variable rates up to day 90. The highest average size of population with 6.2 x104, 2x104, 4.8x103, 4.6x103, and 4.6x103 propagules per each gram of dry soil were related to the treatments of: organic matter 3.2%, pH 4.9, humidity 70%, temperature 25oC, and biological activity of 0% respectively. The same order was recorded more or less for the rates of survival at the end of experiments. They were 45.88, 3.88, 0.51, 0.48, and 0.47 of the initial populations, corresponding to the treatments of organic matter 3.2%, pH 4.9, humidity 70%, biological activity of 0% and temperature of 25oC respectively. A more conspicuous correlation(r — 0/831) was observed between the rate of soil organic matter and the percentage of surviving propagules, this correlation demonstrating a negative value(r — -0/618) for biological activity of soil at the end of 90 days.