The energetics of the ciliary crown-filament pump were studied for the suspensionfeeding polychaete Sabella penicillus. Maxlmum filtration rate expressed as the clearance capacity (F, 1 h-' ind.-') as a function of body slze (W, g dry wt) was: F = 1 3 . 6 2 W~.~~. The filtration rate was high and constant at algal (Rhodomonas sp.) concentrations below about 4 X 103 cells ml-', but at higher concentrations the gut capacity was probably exceeded thus leading to a reduced filtration rate. Oxygen consumption (R, m1 O2 h-' ind.-l) as a function of size was: R = 0.13W'.~~. The water-processing capacity of a 'standard' 65 mg dry wt S. penicillus was estimated as 354 1 of water filtered per m1 of oxygen consumed. This suggests that the polychaete is adapted to live in waters with extremely low algal concentrations. Filtration rate as a function of temperature was measured in 2 size groups of worms. The relationship fitted straight lines and it was found that the viscosity effect may explain the whole correlation between filtration rate and temperatures between 5 and 20 "C. The operating point, 0,, of the crown-filament pump was determined by equating pump characteristic and system characteristic, AHp = AH,. The system characteristic was calculated as the sum of the 2 major contributions, namely the pressure drop across the pinnule-lattice of the crown-filaments, AH,,,, and the kinetic loss, AHkex, in the water leaving each crown-filament, which was regarded as one of a series of parallel 'pump units' The calculated operating point and components for e.g. 15 "C were: 0, = AHipc + AHkex = 0.0222865 + 0.000065 = 0.0224 mm H20. The mechanical work done by the pump (pumping power) was 0.451 yW, compared to a total metabolic energy expenditure of R = 112 pW.