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Seven cultivars of <i>Ocimum</i> spp. were evaluated in greenhouse experiments and field trials for their responses to infection by <i>Peronospora belbahrii</i>, the pathogen of basil downy mildew. Basil leaves were collected and tested for peroxidase (POX) activity and total phenolics (Phe). A significantly (P < 0.0001) different level of POX activity and Phe content was detected among the seven basil cultivars. ‘Lemon basil’ and the cultivar ‘Red rubin’ were found to be resistant with nearly no disease (< 1%) developed until 18-20 days after inoculation (DAI) in the greenhouse inoculations. They also exhibited higher POX activity, higher Phe content and longer period of time from inoculation to symptom occurrence compared to the other five cultivars. Disease severity was lower than 60% on the cultivars ‘Genovese’ and ‘Large leaf Italian’ until 14 DAI, but the cultivars ‘Cinnamon’, ‘Thai’ and ‘Common basil’ had a disease severity up to 85% within 14 DAI. In field trials, lemon basil sustained nearly a disease-free vigorous growth 14 weeks after planting, but ‘Red rubin’ was more prone to the natural infection than in the greenhouse experiments. POX activity in field grown ‘Lemon basil’ was also significantly (P < 0.0001) higher than other cultivars. It is found that POX activity in basil was inversely correlated with downy mildew disease epidemics. POX activity and Phe content of basil cultivars could potentially be used to rapidly assess resistance in basil against downy mildew disease.<p><p>Keywords: Oomycete, Vegetables