Phototrophic growth of Dunaliella tetiolecta Butcher or Pavloca lutheri (Droop) Greet) on hypoxanthine or allantoatc produces major ultrastructural changes, by comparison with nitrate- and urea-grown cultures. These are particularly noticeable during growth on hypoxanthitie and affect mainly the endoplasmic reticulum, mitochondria and vacuoles. Cytochemical tests reveal the presence of both uricaase and cataJase activities in mitochondria. Since no mierobodies are observed m cells of these trncroaluac, it appears that mitochondria participate in the oxidativc degradation of putines and their derivatives. These oryancllcs may therefore play a major role m the organic N budget oi Dunaliella tertiolecta and Pavlova lutheri.