1. 1. The biochemical composition and energy content of the body wall (BW) and pyloric ceca (PC) tissues of Pycnopodia helianthoides were measured for juveniles and adults ranging from 14 to 176mm radius. 2. 2. Juveniles (R = 14-22 mm) had BW tissues composed of 62.5% ash, 1.2% carbohydrate, 32.1% protein and 4.2% lipid. Medium sized adults (R =24-35 mm) had BW values of 57.8% ash, 1.1% carbohydrate, 36.9% protein and 4.2% lipid. Large sized adults (R =85-176 mm) had BW values of 48.9% ash, 1.2% carbohydrate, 44.4% protein and 5.5% lipid. 3. 3. The energy content (kJ/g dry wt) of the BW tissues increased 26% with increasing body size (9.4 kJ/g dry wt, R = 14-22 mm; 10.6 kJ/g dry wt, R = 24-35 mm; 12.7 kJ/g dry wt, R =85-176 mm). 4. 4. The biochemical composition and energy content of the pyloric ceca remained constant over different body sizes (ash = 8.4%; carbohydrate = 3.2%; protein = 73.3%; lipid = 15.1%; 25.2 kJ/g dry wt). 5. 5. Greater amounts of organics and energy in the total BW than the PC tissues and increases in nutrient and energy levels of the BW (kJ/g dry wt) with growth indicates that BW tissue is not biochemically static, and may not only serve a structural function, but provide a reservoir for nutrients and energy. © 1989.