Ophiobolin A [1], 3-anhydroophiobolin A [2], ophiobolin B [3], and ophiobolin L [4] were isolated from fermentation broths of Cochliobolus heterostrophus. Preliminary screening showed that a number of organisms were capable of metabolizing the sesterterpene ophiobolin A [1]. Large-scale transformations of ophiobolin A [1] with Polyangium cellulosum produced 6 and 7 while Pseudomonas aeruginosa produced 8. Resting-cell preparations of Penicillium patulum afforded 9 and 10. The structures of these metabolites were established by spectroscopic methods and by comparison of the spectral data with those of the starting material. The antimicrobial activity of the ophiobolins was also evaluated.