1. Crawley, M. J., Hails, R. S., Rees, M., Kohn, D. & Buxton, J. Nature 363, 620–623 (1993).
2. United States Department of Agriculture, APHIS Biotechnology Permits Unit, database release 12 May 1993.
3. Williamson, M. Experentia (in the press).
4. Ellstrand, N. & Hoffman, C. Bioscience 40, 438–442 (1990).
5. Drake, J. H. et al. Biological Invasions: A Global Perspective (Wiley, New York, 1988).