Downy mildew of rocket (Eruca vesicaria ssp. sativa) and leaf mustard (Brassica juncea) occurred in Tako, Chiba on July 2003. Both causal agents were identified as Peronospora parasitica (Persoon: Fries) Fries. Physiological specialization of both fungi were different from each other and those of already-known P. parasitica. This is the first report on these diseases in Japan.