A turbellarian, probably Amphiscolops langerhansii von Graff, contains yellow-brown algae or Zooxanthellae. Experiments testing the ability of worms to survive in sealed bottles of seawater, when kept in the light and in the dark, indicated that the photosynthesis carried on by the Zooxanthellae was of distinct benefit to the animals harboring them. Measurements of the dissolved oxygen showed that in diffuse daylight a sufficient amount was produced by the algae to maintain a relatively constant oxygen level. In the absence of light a gradual reduction in the amount of dissolved oxygen occurred which was proportional to the number of worms and the time in the dark. The average amount of oxygen produced by the Zooxanthellae of a single Amphiscolops, when kept in bright diffuse daylight, was 0.069 cc. per liter per day.