The life cycle of Codium fragile shows strong seasonal trends with respect to frond morphology and chloroplast physiology. The period of maximum carbon fixation, pigment content and chloroplast size occurs during the early winter when competition from other algae is small and the variation in tidal amplitude decreased. At this time of year the alga may be able to accumulate nutrient reserves as evidenced by the seasonal variation in chloroplast alkali: acid-soluble polyphosphate ratios. In the summer when the physical environment is more extreme (due to prolonged tidal exposure) and competition from other algae greater, photosynthetic rates, pigment concentration and chloroplast size are reduced and the alga enters the reproductive phase. This part of the life cycle is accompanied by the development of frond hairs which may increase nutrient uptake and the mobilization of stored polyphosphates sequestered during the winter vegetative phase. It is proposed that these seasonal trends in frond morphology and chloroplast physiology may constitute an ecologically advantageous growth strategy in C. fragile.