The pathways of carbon flow during photosynthesis and photorespiration by marine microalgae are examined. It is concluded that, although in healthy, exponentially dividing cells of marine phytoplankton the bulk of carbon assimilation is via ribulose-1,5-bisphosphate carboxylase and the photosynthetic carbon reduction cycle, a significant proportion of carbon fixation may be by β-carboxylation (via phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase or pyruvate carboxylase). However, such carbon assimilation does not represent a "C4 pathway" as found in certain higher plants. The fixation of carbon via β-carboxylation may, however, be a significant route of carbon assimilation in low irradiance environments. Although many microalgae, both marine and freshwater, posses "C3 biochemistry", they nonetheless have physiological characteristics resembling those of C4 higher plants. The available evidence suggests that this is a consequence of the operation, in microalgae, of a "CO2 concentrating mechanism" which, effectively, suppresses photorespiration. However, the capacity for carrying out photorespiratory carbon metabolism is present in such cells even though photorespiratory activity may be reduced. The limited evidence available suggests that marine microalgae may possess the enzymatic machinery for both the glycolate pathway and, in diatoms at least, the tartronic semialdehyde cycle of photorespiratory carbon metabolism. The possession of a mechanism to accumulate CO2 and suppress photorespiration could confer a competitive advantage under conditions of CO2 limitation and, possibly, under nitrogen limitation.