Three new oxindole derivatives, RK-270A (1), B (2) and C (3) (Figure 1), were discovered and isolated from Streptomyces sp. RK85- 270. They had an isopropylidene group at C-3 position of an oxindole skeleton, and C-1 also had a prenyl group and belonged to a class of 6-prenylated indoles. The isolation of this type of indoles was the first example as a natural product. Their cytotoxicity and antibacterial activities were evaluated.