Earthworms are one of the most important groups of invertebrates in the soil. They are also considered as ecosystem engineers because of their effects on soil properties. Different ecological categories of earthworms act differently on the activity of microorganisms. The influence of epigeic earthworms Eisenia fetida on dehydrogenase activity in soil was investigated during six-month incubation of the microcosms which had a sterile and unsterile soil treatments. Dehydrogenase activity in earthworm's gut contents was also investigated during six-month incubation period. According to the changes of the enzyme activity under the presence of the earthworms, changes in the physical (dry weight, ash weight, and ash free dry weight) and chemical (pH value, electrical conductivity) soil properties were also expected. Dehydrogenase activity is used as an indicator of overall soil microbial activity because they occur intracellular in all living microbial cells. Soil dehydrogenases belong to the oxidoreductase enzyme class and play a significant role in the biological oxidation of soil organic matter. In this study, dehydrogenase enzyme activity was determined by the reduction of INT (iodonitrophenyl tetrazolium chloride) to INTF (iodonitrophenyl tetrazolium formazan). The results showed that earthworms had a significant effect on the increase or decrease of the enzyme activity in the investigated treatments, depending on the period of incubation. It is also shown that dehydrogenase activities in earthworm's gut contents were significantly lower than those in the surrounding soil.