The population dynamics of indigenous soil bacteria and protozoa on decaying barley roots were followed by using litter bags buried in laboratory-incubated soil. The soil was either non-treated or treated with the fungicide fenpropimorph (in the formulation Corbel) at concentrations corresponding to the recommended and at 10 times field dose (1.3 and 13 mg kg^-1 dry wt.). Number of total bacteria and number of Pseudomonas were detected, using both traditional plating and short-time incubations of ‘early' colonies, to determine the fast-responding subpopulation of the culturable bacteria. The number of protozoa corresponding to the two subpopulations was followed. The results strongly indicate a predatory association between the protozoa and bacteria. This was shown by a tight temporal association, and by a stimulation of bacteria following predatory release when protozoa were inhibited by fenpropimorph. Thus, fenpropimorph disturbed population dynamics in concentrations, which can be reached in surface soils after distribution in the field.