We evaluated the changes of some soil microbiological characteristics due to the use of transgenic maize expressing Bacillus thuringiensis (Bt) toxin. A two-year field experiment was conducted (2003 and 2004). Two lines of transgenic Bt maize that express the Cry1Ab protein (event 176 and MON 810) and their near-isogenic non-Bt lines were used. Rhizosphere and non-rhizosphere soils were collected and measurements were performed during the maize cultural cycle and immediately at pre-harvest. Key soil microbiological parameters measured included the numbers of culturable aerobic bacteria, including actinomycetes, and fungi, the activity of dehydrogenase and nitrogenase enzymes and ATP content. There were clear seasonal effects in the microbial parameters as evidenced by the consistent changes in sampling dates across the two years. Differences in the measured variables were also observed between rhizosphere and non-rhizosphere soils. However, under our field conditions, the presence of Bt maize did not cause, in a general way, changes in the microbial populations of the soil or in the activity of the microbial community.