Actinomycete strains OM-6519(T) and K04-0144(T) produce the bioactive compounds lactacystin and cyslabdan, respectively. Here, the taxonomic positions of these two strains were determined. The morphological and chemical features of strains OM-6519(T) and K04-0144(T) indicated that they belonged to the genus Streptomyces. Strain OM-6519(T) showed the highest 16S rRNA gene sequence similarities with Streptomyces xanthocidicus NBRC 13469(T) (99.7%), Streptomyces chrysomallus subsp. fumigatus NBRC 15394(T) (99.6%) and Streptomyces aburaviensis NRRL B-2218(T) (99.5%). However, the DNA-DNA relatedness values between strain OM-6519(T) and the three related strains were below 70%. Strain K04-0144(T) showed the highest 16S rRNA gene sequence similarities with Streptomyces corchorusii NBRC 13032(T) (99.4%), Streptomyces olivaceoviridis NBRC 15394(T) (99.4%) and Streptomyces canarius NRRL B-2218(T) (99.3%). However, the DNA-DNA relatedness values between strain K04-0144(T) and the three related strains were also below 70%. Based on morphological, cultural and physiological characteristics and DNA-DNA relatedness data, strains OM-6519(T) and K04-0144(T) should be classified as new species of the genus Streptomyces, for which the names Streptomyces lactacystinicus sp. nov. and Streptomyces cyslabdanicus sp. nov. are proposed. The type strain of S. lactacystinicus is OM-6519(T) (=NBRC 110082(T), DSM 43136(T)). The type strain of S. cyslabdanicus is K04-0144(T) (=NBRC 110081(T), DSM 42135(T)).