The growth kinetics of a vacuolated Spirulina platensis strain BP and a non-vacuolated Spirulina platensis strain 12D in open pond cultures were studied. The changes in the dissolved oxygen concentration of the cultures were parallel to the sunlight intensity profiles during the day. The specific growth rate remained unchanged over a large part of the day, although the growth rate in the early morning varied as a function of cell concentration. The variation in the dark respiration rate during the day was about 30%. The photosynthesis of the cultures was not photoinhibited in the early morning. The degree of photoinhibition increased in the morning to reach a value of 15-24%, depending on the cell concentration and light intensity at noon, and then decreased gradually in the afternoon. The study suggests that the rate limiting step in the growth of Spirulina cultures using sunlight was not at the light harvesting step (photosynthesis), but the rate of uptake of a key substrate, or the activity of a key enzyme in the biochemical pathways.