Abstract A Montana soil actinomycete, Streptomyces anulatus, produced (1×10 −2% yield) a new cancer cell growth inhibitory cyclooctadepsipeptide named montanastatin ( 1) accompanied by the potent anticancer antibiotic valinomycin ( 2) in very high (5.1%) yields. Valinomycin but not montanastatin inhibited growth of a number of pathogenic bacteria and fungi. Interpretation of high-field (500 MHz) NMR and high-resolution FAB mass spectral data allowed assignment of the structure cyclo-( d-Val- l-Lac- l-Val- d-Hiv) to montanastatin. Valinomycin ( 2) was also isolated from actinomycetes cultured from a tree branch and animal feces collected in Malaysia. Streptomyces exfoliatus, isolated from the tree branch, was found to contain valinomycin in 1.6% yield, while the fecal isolate, S. anulatus, gave valinomycin in 0.9% yield.