Technical Abstract: Fifty-two isolates of Plasmopara halstedii Farl. Berl. & de Toni (causal agent of sunflower downy mildew collected from sunflower (Helianthus annuus L.) in Spain during 1994-2000 were evaluated for metalaxyl resistance. Apron 20% LS was applied to seed of susceptible sunflower `Peredovik' at 2.0 g a.i./ kg seed. Two replications of 40 plants were established and inoculations were repeated three times. Two-day-old untreated and treated seedlings were dipped in a suspension of 60 x 103 sporangia/ ml for 4 h, planted in sand and perlite (2:3 vol/vol) and grown at 16-21 C with a 12-h photoperiod. After 12 days, sporulation and systemic chlorosis were evaluated in plants. Metalaxyl provided total control of 51 isolates. The remaining isolate caused symptoms in 67% of treated plants. This isolate was tested in another experiment where `Peredovik' seed was treated with metalaxyl at doses of 0, 0.5, 2.0, 3.5 and 5 g a.i./kg seed, with four replications of 12 seedlings/ treatment. The incidence of downy mildew was similar on all treated plants, except at the highest rate, which is twice the labeled European rate. This is the first report of resistance of sunflower downy mildew to metalaxyl in Spain.