An analysis of mechanism why bacteria were essential for growth of Uroglena americana was conducted by use of a temporary axenic culture. Addition of culture filtrate of bacteria to the algal culture was ineffective. Dialysis cultivation of U. americana and bacteria, eliminating cell-to-cell contact, resulted in complete loss of the algal growth. Heat-killed bacteria supported the algal growth, suggesting phagotrophic feeding of bacteria in this alga. Microscopic examination re-vealed that U. americana actively engulfed tetrazorium-stained bacteria.