The data presented for three strains of Euglena gracilis corroborate previous reports of a diel rhythm in photosynthetic capacity in divisionsynchrnized cultures of this alga and extend these studies to free nning, dividing and nondividing (stationary) Circadian rhythms in photosynthesis have been observed in a number of higher plants as well as in algae (16). Several studies have reported a rhythm of photosynthetic capacity during the cell cycle of the alga flagellate Euglena (3,4,22,32), although the results are often conflicting. Walther and Edmunds (32) demonstrated a clear 24-h rhythm in the capacity of Euglena to fix CO2 in cultures synchronized by appropriately chosen light (12,000 lux) cycles (LD: l0,14 This lack of long term persistence of the photosynthetic capacity rhythm and hence the inability to conclude that an endogenous circadian clock underlies the overt periodicity, could have been

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