A marked seasonal response was observed in the starch content of chloroplasts of Codium fragile sampled between November 1983 and November 1984. Chloroplast starch uniformly increased throughout the life-cycle of this alga, reaching a maximum in the summer months of approx. 5 mg glucose equivalents/109 plastids when C. fragile was in the reproductive phase. Chloroplast starch content was found to be inversely proportional to the concentration of stromal phosphate, decreasing linearly from 5 to 2 mg glucose equivalents/109 plastids over a range of stromal phosphate values of 0·75–1·5 μmol PO4/109 plastids. Similarly, the net amount of starch synthesized over 3 h by isolated chloroplasts increased linearly with respect to initial chloroplast starch content. These observations are discussed in relation to the regulation of starch synthesis by variations in the stromal P1: PGA ratio.