To date, Lemna and Spirodela species of the family Lemnaceae are the only species that have been used to produce recombinant proteins for pharmaceutical and veterinary purposes. Wolffia arrhiza is the most evolutionarily advanced species of the Lemnaceae. A rootless duckweed, it is the most promising target for biopharmingas a candidate for submerged cultivation in a fermenter. As a first step toward future biotechnological use of Wolffia, we established a stable transformation system for it based on Agrobacterium-mediated transformation. Following inoculation with the bacteria, Wolffia cluster explants were cultured for 2 weeks on media containing 2.0 mg l(-1) 2,4-dichlorophenoxyacetic acid and 2.0 mg l(-1) N-6-benzyladenine. Explants were then transferred to growth regulator-free media in the presence of 5.0 mg l(-1) hygromycin B to select antibiotic-resistant plants. Other selective agents-kanamycin and phosphinothricin-were not suitable for Wolffia, nor was application of particle bombardment for the delivery of foreign DNA to Wolffia explants. The developed agro transformation conditions yielded stably transformed lines of Wolffia, confirmed by Southern blotting, with an efficiency of 0.2-0.4 % transgenes per 100 explants.