Work previously published by the author shows that either cyclic photophosphorylation or oxidative phosphorylation can supply ATP to a variety of ATP-requiring processes in Hydrodictyon africanum. The rate at which the process occurs with ATP supplied from cyclic photophosphorylation is in some cases faster than that when ATP is supplied by oxidative phosphorylation; for other processes the reverse is true. The reasons for this variability in the rate at which the two ATP sources can support different ATP-requiring processes is discussed and it is suggested that regulation of the rate of ATP-requiring processes involves changes in the activity of the ATP-consuming reactions as well as in the ATP concentration (or energy charge). It is concluded that the in vivo capacity for cyclic photophosphorylation is at least as great as that for oxidative phosphorylation. Based on the capacity for respiratory electron flow in vivo, it is suggested that cyclic and oxidative phosphorylation each have capacities of some 20 μMoles (mg chl)−1 h−l. Based on the capacity for non-cyclic electron flow in vivo, it is suggested that the capacity for non-cyclic phosphorylation in vivo is 150 μMoles (mg chl)−1 h−1. Cyclic photophosphorylation saturates at an irradiance of about 1 J m−2 s−1, while non-cyclic photophosphorylation saturates at about 10 J m−2 s−1. These low figures for this shade-Actapted alga contrast with about ten- to twenty-fold higher values for capacity and light requirement for the various phosphorylation processes in sun-Actapted microalgae.