Two species of heliobacteria along with a purple and green bacterium were tested for their ability to grow phototrophically at irradiances ranging from 0.125 to 50 W m-2. The heliobacteria were incapable of growth below 0.5 W m-2, while both the purple and green bacterium grew at significantly lower irradiances. Specific bacteriochlorophyll contents were higher for the purple and green bacteria than for the heliobacteria at all irradiances tested. Thus in distinct contrast to purple and green bacteria, heliobacteria are "high-irradiance" phototrophs, and this characteristic may influence their distribution in nature. and L. K. Kimble-Long, M. T. Madigan.