Algae are rare within the bacterial blooms of Leptothrix ochracea that occur within streams possessing zones of iron and manganese oxide deposition. To assess the importance of L. ochracea and metal oxides for constraining algal abundance, and to elucidate the role these bacteria have in maintaining oxide deposition zones, we conducted field studies in a stream containing an extensive bloom of L. ochracea. Transplant experiments revealed a strong, inverse relationship between percent cover of ferro-manganese oxides and the number of algal cells on colonized microscope slides. This negative relationship held whether substrata were colonized first with algae and then placed within the bloom, or were colonized with L. ochracea and then moved outside the bloom; in either case oxide cover was correlated with reduced numbers of algal cells. In another experiment, Leptothrix-bearing slides that had been sterilized with formalin had significantly more algae than did unsterilized controls. Ferro-manganese oxides were more prevalent on unsterilized slides, indicating the bacteria have an active role in maintaining ferro-manganese oxides within the bloom. Active deposition of metal oxides may benefit bacteria by: 1) deterring algal colonization via allelopathic action, and 2) adsorbing phosphorus and depriving algae of this essential nutrient. Results from this study suggest that an antagonistic relationship exists between stream algae and L. ochracea and that both bacterial activity and the existence of ferro-manganese oxides mediate the balance between these organisms in nature.