A c o m b i n a t i o n of a direct and an i n d i r e c t m e t h o d has been used for the first time in the study of f i l t e r -f e e d i n g in benthic invertebrates. E x p e r i m e n t s over 12 h periods under c o n s t a n t temperature, pH, light and feeding conditions have been carried out with 3 species of ascidians: Ciona intestinalis (Linn&, 1767); Phallusia mammillata (Cuvier, 1815) and Styela plicata (Lesueur, 1823). In C. intestinalis and P. mammillata, no p u m p i n g r h y t h m has been found; the w a t e r flow was c o n s t a n t for 12 h, except for some brief accidental interruptions. In s. plicata, the p a r t i c u l a r rhythm of s p o n t a n e o u s p e r i o d i c a l c o n t r a c t i o n s r e c o r d e d during the e x p e r i m e n t were related to gamete emission. The speed of w a t e r flow, w h i c h was fairly c o n s t a n t during recording, v a r i e d c o n s i d e r a b l y during the 12 h period; these variations were generally related to m o d i f i c a t i o n s in the d i a m e t e r of the cloacal siphon. The average speeds, in cm sec -I , lay b e t w e e n 5.7 and 9.5 for c. intestinalis, 5.2 and 19.4 for P. mammillata, and b e t w e e n 5.3 and 10.7 for s. plicata. V a r i a t i o n s in the p u m p i n g -r a t e were small, o s c i l l a t i n g a r o u n d a mean value. In ml h -I g,1 dry w e i g h t of organs these were 5,829 to 5,982 (mean = 5,906) for c. intestinalis, 6,142 to 6,592 (mean = 6,312) for p. mammillata, 1,O508 to 1,1505 (mean = 1,0708) for S. plicata. F i l t r a t i o n was continuous w i t h o u t any p a r t i c u l a r rhythm; f i l t r a t i o n rates in ml h -I g-1 dry weight of organs v a r i e d b e t w e e n 4,244 and 4,418 (mean = 4,331) for c. intestinalis, 4,620 and 4,960 (mean = 4,779) for P. mammillata, 8,482 and 9,078 (mean = 8,760) for s. plicata. The curves r e p r e s e n t i n g p u m p i n g and filtration were cle&rly parallel, i n d i c a t i n g that filtration e f f i c i e n c y did not vary greatly during the course of an experiment; the rates o b t a i n e d were 65 to 87% (mean = 74%) for c. intestinalis, 66 to 88% (mean 76%) for P. mammillata, and 73 to 90% (mean = 80%) for s. plicata. The slightly h i g h e r m e a n value in s. plicata is p r o b a b l y related to the h i g h e r c o m p l e x i t y of the b r a n c h i a l apparatus of this species.