Zhou, X. and Wu, F. 2013. Artificially applied vanillic acid changed soil microbial communities in the rhizosphere of cucumber (Cucumis sativus L.). Can. J. Soil Sci. 93: 13-21. Phenolic acids (PAs) have been implicated as autotoxins of cucumber (Cucumis sativus L.), but doubts about the importance of these compounds also exist due to their low concentrations in the field. The physiological alterations caused by PAs are concentration dependent, and the range of bioactivity is between 0.1 and 1 mM. Here, vanillic acid (VA) (0.02, 0.05, 0.1, 0.2 µmol g^-1 soil) was applied into the soil every other day for five times. The effects of VA on C. sativus seedling growth and rhizosphere soil microbial communities were evaluated. Soil bacterial and fungal community structures and sizes were analyzed with polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) and real-time PCR methods, respectively. Vanillic acid significantly inhibited C. sativus seedling growth at concentrations ≥ 0.05 µmol g^-1 soil, and stimulated soil dehydrogenase activity, soil microbial biomass carbon content, and soil bacterial and fungal community sizes at all concentrations tested. Vanillic acid also caused shifts in rhizosphere soil bacterial and fungal community structures. Overall, VA could change rhizosphere soil microbial communities even when the VA concentration was lower than the level that produced a significant phytotoxic effect in cucumber.