The biochemical composition of the cyprids of Balanus balanoides is described. The mean total freeze-dried weight per cyprid was 37.7 μg. Mean values, as percentages of total dry weight were 9.2% neutral lipid, 4.8% phospholipid, 0.8% polysaccharide, 2.7% free sugars, 6.8% protein nitrogen, 8.2% total nitrogen, 3.2% chitin and 12.4% ash. Free-swimming cyprids, prevented from settling for up to 8 weeks at 8°C utilised their neutral lipid (triglyceride) reserves. At the end of this period 90% of the initial neutral lipid reserves were depleted. There was only a slight decrease in the phospholipid, free sugar and nitrogen content of the cyprids, and the polysaccharide content remained comparatively constant.