The single‐celled algaChlorella vulgariswas cultured in the liquid phase of pig slurry under two different light intensities, and the development of the indigenous bacterial flora was compared qualitatively and quantitatively with that of controls not inoculated with chlorella. Particular attention was given to the occurrence and survival of potential pathogens. Algal growth was found to favour the growth of certain bacteria but had little effect on or was hostile to that of others. The effects of light were mediated by the algae rather than having a direct effect on the bacterial cells. The algae, which were harvested by centrifuging and then freeze‐dried, were found to enhance the survival of certain bacteria during this proced