The Ministry for Environment, Health, and Consumer Protection in the German state of Bavaria funded a cooperative research network entitled Assessment and Monitoring of Genetically Modified Organisms Placed on the Market. Within this framework, the Bavarian State Research Centre for Agriculture (LfL) evaluated and monitored the environmental effect of Bacillus thuringiensis (Bt) maize from 2000 to 2004. The LfL research focussed on potential side effects on nontarget organisms including aphids and antagonists, butterflies, soil micro-organisms, springtails, earthworms, and nematodes in the laboratory, small field blocks, and fields of commercial size. Of that research, only the studies on nematodes are presented in this article. As an abundant and diverse group in agricultural soils, nematodes are an important part of the soil/food network and have functional diversity in agrarian ecosystems. They may be relevant bioindicators, as plant-feeding nematodes could be affected directly by Bt toxin from the maize tissues, while saprophagous nematodes may be indirectly affected by changed fungal or bacterial food sources. A more detailed and comprehensive final report was published as a special volume in the series on all LfL project results (http://www.LfL.bayern.de).