Effects of dark time and of oxygen supply upon photosynthetic induction in Ulva were investigated polarographically and manometrically. Under continuously aerobic conditions, the induction period was of about two minutes' duration following one day in darkness in flowing sea water; beyond 10 min., length of the aerobic dark period had little effect. The induction period was lengthened by dark anaerobiosis, but was not eliminated entirely by dark aeration or oxygenation treatments. Aerobic induction periods of similarly brief duration were observed in Ulva, Gigartina, and Gelidium fronds which corresponded with those established previously for leaves and unicellular algae. Aerobic induction is attributed to an initial lack of "CO2-acceptor" which is regenerated photosynthetically. Interpretations are provided for the widely different induction periods and saturating carbon dioxide concentrations which had been observed in earlier studies of photosynthesis in attached marine algae.