Biofilms form rapidly on surfaces in contact with the water reclamation systems proposed for long-term space travel. These microbial populations pose a potentially serious hazard to the astronauts, the engineering systems and to the function of plant growth chambers.Human pathogens present in grey water survive for long periods on surfaces, protected against disinfection by the presence of a biofilm matrix. In addition, some of the materials being proposed for use in water reclamation systems for the orbiting space station are highly susceptible to microbial degradation. Both blockage and corrosion can occur as a result of the interactions between the microorganisms and the materials.Chemicals produced by microbial processes during growth on surfaces include both metals and organic toxins. These by-products of metabolic activity are potential hazards to astronaut health and growth of plants being considered as part of the life support system. Preliminary data indicate that metals are released into solution as a result of biofilm growth. Similarly, toxic chemicals are leached from organic materials coated with microorganisms. A better understanding of the importance of microbial contamination of water reclamation systems will permit development of improved control measures for maintenance of astronaut health and life support systems during long-term space travel.