Genetically modified (GM) crops may affect earthworms either directly through the plant, its root exudates and litter, or indirectly through the agricultural management changes that are associated with GM plant production.  In order to investigate such possible effects, we established two field studies of Bacillus thuringiensis (Bt) maize and a glufosinate ammonium tolerant maize and included a reduced tillage (RT) treatment and a conventional tillage (CT) treatment as examples of a likely concomitant change in the agricultural practise.  At a French study site at Varois, (Bourgogne), a field grown with the Bt-toxin producing transgenic maize line MON810 was studied for 1 year.  At a Danish study site, Foulum (Jutland), 1 year of Bt maize was followed by 2 years of herbicide tolerant (HT) maize.  At the French study site, the most prominent effects observed were due to the tillage method where RT significantly reduced the earthworm populations to levels about half of CT.  At the Danish study site effects of CT complied with known reduction of anecic earthworms due to this technique and likewise effects of RT were observed for endogeic earthworms.  Earthworm populations were diminished with the HT crop, probably due to exposure to the herbicide Basta® during the two consecutive autumn seasons.  This study confirms the importance of including the tillage techniques and pesticide usage when evaluating the environmental effects of new agricultural technologies.