The fine structure of iron deposits produced byLeptothrixspp. from two caverns and a surface spring were observed. In the case ofLeptothrix pseudovacuolata,the iron occurred as an amorphous deposit within the cell sheath and the cells contained gas vesicles. In the case of the otherLeptothrixspp., the iron was deposited within the cell sheath either as a hexagonal or fibrillar matrix. In the case of fibrillar iron deposits, fibrils resembling those of the sheath were found within the cytoplasm of the cells. This suggests that the deposition of iron could have occurred within the cells as well as at the cell surface. In some cases, the fine structure of microbial iron deposits could provide a means of distinguishing biological from abiological iron deposition.