Acidithiobacillus ferrooxidans R 1 and Desulfovibrio vulgaris NCIMB 8457 were taken as test strains in a study for the development of a chemotaxis test using a modified capillary assay. 2.0-mL glass capillaries were filled with 1.6 ml of a soln. contg. chemotactic substances under vacuum. Thereby a better load of the capillaries with the chemotactic substances was ensured. This enabled a highly precise, direct microscopic enumeration of cells using a THOMA chamber. Attractants were characterized by a migration of cells into glass capillaries filled with chemotactic substances and consequently high cell nos. therein. Compared to a control, a low cell no. in the glass capillaries indicated that the test substance was a repellent. A. ferrooxidans was attracted by Fe2+ ions and tetrathionate. The effect was dependent on the concn.: at a concn. of 100.0 mM Fe2+, 6.0 * 103 cells/capillary were enumerated, at a concn. of 1.0 mM Fe2+, only 103 cells/capillary were counted. The effect of tetrathionate was not as significant as that of Fe2+ ions: at concns. of 1.0 and 10.0 mM tetrathionate, 1.0 * 103 cells per capillary and at a concn. of 100.0 mM tetrathionate, 2.0 * 103 cells/capillary were detd. After the application of 0.1, 1.0 and 10 mM aspartate, low cell nos. in the glass capillaries were found. Consequently, aspartate acted as a repellent. Cells of D. vulgaris were attracted by lactate and thiosulfate. At a concn. of 10.0 mM lactate, the highest amt. of bacterial cells (1.6 * 105 cells/capillary) was measurable. Between 8.0 * 104 and 1.0 * 105 cells/capillary were detd. at concns. of 100.0, 1.0, 0.1 and 0.01 mM lactate. The effect of thiosulfate was not as significant as the effect of lactate: at a concn. of 10.0 mM thiosulfate, 5.0 * 104 cells/capillary, at a concn. of 1.0 mM thiosulfate, 3.0 * 104 cells/capillary and at a concn. of 0.1 mM thiosulfate, 2.0 * 104 cells/capillary were detd. Finally, the effect of the highly toxic biocide tributyltin chloride was tested. The cell nos. of D. vulgaris were not affected by concns. of 0.01 and 0.1% tributyltin chloride compared to the control assay (4.1 * 104 and 3.8 * 104 cells/capillary). This result is important for the application of tributyltin chloride compds. Consequently, the use of the modified capillary assay enabled fast tests and yielded reproducible results for measurements of chemotactic activities of Acidithiobacillus ferrooxidans and Desulfovibrio vulgaris. Dependent on the concn., the impact of biocides or inhibitory substances on the main bacteria involved in bioleaching and corrosion processes could be detected as a matter of routine.