Fifteen Peronospora tabacina isolates were compared for sensitivity to dimethomorph and metalaxyl using a floating leaf disc assay in duplicate experiments. Thirteen of the isolates were collected from tobacco fields in North Carolina in 1997, and one each was from North Carolina in 1995 and Kentucky in 1979. Leaf discs (18 mm diam) were cut from 7-wk-old Burley 21 tobacco plants and ten discs each were floated upside down on 20 ml solutions containing 0.0, 1.0, 2.5, 5.0 and 10 µg/ml dimethomorph and 1.0 and 100 µg/ml metalaxyl in separate 100 X 15 mm petri dishes. The abaxial surface of the leaf discs in separate dishes was inoculated with the different isolates by spraying 1 ml of a sporangia suspension (20,000 sporangia/ml) per dish. Leaf discs were incubated at 21°C (12 hr light)/18°C (12 hr dark) and examined after 8 days for sporulation. Sporulation for all fifteen isolates was profuse in the controls and was completely inhibited at 1.0 µg/ml dimethomorph. Sporulation of the 1979 isolate was completely inhibited at 1.0 µg/ml metalaxyl and the remaining fourteen isolates sporulated at 100 µg/ml metalaxyl. Additional leaf disc experiments were conducted using the 1979, 1995 and three of the 1997 isolates and dimethomorph concentrations of 0.0, 0.1, 0.5, 1.0, 5.0 and 10 µg/ml. In these experiments, the minimum inhibitory concentration (mic) of dimethomorph for P. tabacina was 1.0 µg/ml. There was no apparent difference in sensitivity to dimethomorph between the 1979, 1995 and 1997 P. tabacina isolates tested. These studies are an indication that the baseline mic for P. tabacina using the floating leaf disc assay is approximately 1.0 µg/ml dimethomorph. Isolates from a larger geographic area should be tested to verify these results.