Imidacloprid is a systemic insecticide which gives good control of aphids on sugar beet when applied as a seed-pellet treatment, or as a granule alone or mixed with carbofuran. It does not give rapid knockdown, but nevertheless prevents plant damage and virus infection during the pre-mortality phase. Applied as a seed treatment, rates from 30 to 90 g a.i. per unit of seed gave equivalent activity in the laboratory against Myzus persicae that were moderately resistant (R1) and highly resistant (R2) to other insecticides. In the field, the 90 g rate controlled naturally occurring aphids for longer than either the 45 g rate, or the granule applied at 600 g a.i./ha, resulting in the best control of virus yellows. The seed treatment at 90 g/unit, the granular formulation at 375 g a.i./ha and the imidacloprid plus carbofuran mixture gave better activity against R2 than R1 M. persicae when the aphids were caged on fieldgrown plants in 1991. Although field trial results did not suggest differential control of the two viruses causing yellows, a laboratory study showed that imidacloprid seed treatment (at 45 and 90 g a.i./unit) reduced infection by BMYV but had no effect on BYV. Imidacloprid applied as a seed treatment offers a more environmentally acceptable alternative for aphid and virus yellows control in sugar beet than the currently available granules and sprays