Microbial transformation studies of the marine diterpene cyanthiwigin B (1), isolated from the Jamaican sponge Myrmekioderma styx, were accomplished. Two actinomycete cultures, Streptomyces NRRL 5690 and Streptomyces spheroides, significantly metabolized cyanthiwigin B to new metabolites. Streptomyces NRRL 5690 transformed cyanthiwigin B to three new compounds, cyanthiwigins AE (2), AF (3), and AG (4), and the known cyanthiwigin R (5). S. spheroides transformed cyanthiwigin B to cyanthiwigins S (6), E (7), and AE (2). All microbial-metabolized derivatives (2-7) of cyanthiwigin B exhibited the ability to increase the antimicrobial activity of curcuphenol, the major antimicrobial sesquiterpene isolated from M. styx.