MANY examples are known of interspecies relationships between widely different organisms in natural communities1. The benefits that one or more members of a population obtain as a result of their association have been described mainly in terms of the nutritional value of the relationship, measured in terms of growth rate, metabolic activities, or the ability to survive in a selective environment1–3. We have discovered an apparent commensal relationship between the alga Chlamydomonas reinhardtii y-1 and a bacterium, tentatively identified as a coryneform bacterium. As a result of their association, the synthesis of a single protein is stimulated to a remarkable extent within the bacterial cells, whereas the algal cells apparently obtain neither benefit nor detriment from the relationship.