CONSIDERABLE data are now available on the radiosensitivity of cultivated plants1,2, but very little is known about the tolerance of wild species. We have investigated the doses of γ-radiation needed to prevent weed seeds from growing. The seeds were stored at 32 per cent relative humidity and irradiated in air with a range of doses from 0 to 100,000 rads of γ-radiation from cobalt-60. They were then planted in steam-sterilized soil and the number of plants arriving at maturity were counted. LD50 values for some common species of arable weeds are listed in Table 1.