Abstract: A photo bioreactor for the growth of photosynthetic organisms comprising a tank (2) for holding a liquid medium, six rotational agitators (3a-3f) for agitating the liquid medium. Each agitator is arranged to agitate about a substantially vertical axis (6a-6f) by sweeping out, upon rotation, a substantially circular path (C) that has a diameter (D) that is substantially coincidental with a radius (R) of the tank (2). Each agitator (3a-3f) is located on a radius which is spaced approximately 60° apart from the next. This allows for more even mixing and more uniform light distribution within the tank.