The influence of long-term mixed organic and inorganic contamination on soil microbial activity, community structure and genetic diversity was investigated in soil samples from a coke oven plant located in Upper Silesia, Poland. The tested soils were heavily contaminated with polycyclic aromatic hydrocarbons (PAHs) and heavy metals. The microbial communities were characterized using the phospholipid fatty acid (PLFA) and the denaturing gradient gel electrophoresis (DGGE) methods. Analysis of the PLFA profiles showed that the biomass of Gram-negative bacteria and fungi was affected by heavy metals but not by PAHs. Similar results were obtained for total microbial activity measured as the rate of fluorescein acetate hydrolysis. Statistical analysis of the obtained results revealed that heavy metals rather than PAHs were primarily responsible for the reduction in microbial activity and the differences in the microbial community structure as showed by PLFA. The DGGE analysis showed that the most contaminated soil had a very low biodiversity and richness but a very high evenness index. The correlation analysis revealed that the biodiversity and richness indices were negatively correlated with PAHs but not with heavy metals. However, there was a positive relationship between the evenness index and tested metals as well as the PAH content. The partial 16S rRNA sequence analysis showed that some of the clones were closely related to the genera Pseudomonas, Sphingomonas and Arthrobacter, which are well-known hydrocarbon degraders. Obtained results indicated that a high level of contamination suppress the some bacterial community member giving finally a reduction in the genetic diversity.