When the marine Chlorophycean flagellate Dunaliella tertiolecta Butcher was grown with short photoperiods of bright light, the use of ammonia rather than nitrate as a nitrogen source led to a 30 % reduction of the doubling time of cell matter. The cell cycle (onset of light to completion of cell division) was shortened by about 10% only. Ammonia-grown cells possessed a greater capacity for photosynthetic oxygen evolution at light saturation than did nitrate-grown cells; their content of ribulosediphosphate carboxylase was likewise greater. The faster growth of Dunaliella tertiolecta with ammonia may be partly a consequence of a general increase in net protein synthesis resulting in a greater content of photosynthetic enzymes.