Nitrogen fixation is one of the most important roles played by soil bacterial communities, as fixation supplies nitrogen to many ecosystems which are often N limited.  As impacts on this functional group of bacteria might harm the ecosystem's health and reduce productivity, monitoring that particular group is important.  Recently, a field trial with Bt white spruce, which constitutively expresses the Cry1Ab insecticidal toxin of Bacillus thuringiensis, was established.  The Bt white spruce was shown to be resistant to spruce budworm.  We investigated the possible impact of these genetically modified trees on soil nitrogen-fixing bacterial communities.  The trial consisted of untransformed controls, GUS white spruce (transformed with the ß-glucuronidase gene), and Bt/GUS white spruce (which constitutively expresses both the Cry1Ab toxin and ß-glucuronidase) in a random design.  Four years after planting, soil samples from the control and the two treatments from plantation as well as from two natural stands of white spruce were collected.  Diazotroph diversity was assessed by extracting soil genomic DNA and amplifying a region of the nitrogenase reductase (nifH) gene, followed by cloning and sequencing.  Analysis revealed that nitrogen-fixing communities did not differ significantly among the untransformed control, GUS white spruce, and Bt/GUS white spruce.  Nevertheless, differences in diazotroph diversity were observed between white spruce trees from the plantation site and those from two natural stands, one of which grew only a few meters away from the plantation.  We therefore conclude, in the absence of evidence that the presence of the B. thuringiensis cry1Ab gene had an effect on diazotroph communities, that either site and/or field preparation prior to planting seems to be more important in determining diazotroph community structure than the presence of Bt white spruce.