Laboratory experiments demonstrated that fungal degradation of lubricating grease produced organic acids and localized corrosion of carbon steel cables in polyvinyl chloride sheaths. Fusarium sp., Penicillium sp. and Hormoconis sp. isolated from corroding tendons in a post-tensioned structure were used in the testing. In all cases when fungal spores were intentionally introduced to sheathed tendons, localized corrosion was observed and there was a spatial relationship between fungal hyphae and corrosion. Grease degradation and concomitant acid production were documented with Fourier transform infrared spectroscopy. The association of fungi with corrosion products and the details of the corrosion were documented with scanning electron microscopy.