Sponges are considered to be filter feeders like their nearest protistan relatives, the choanoflagellates. Specialized "sieve" cells (choanocytes) have an apical collar of tightly spaced, rodlike microvilli that surround a long flagellum. The beat of the flagellum is believed to draw water through this collar, but how particles caught on the collar are brought to the cell surface is unknown. We have studied the interactions that occur between choanocytes and introduced particles in the large feeding chambers of a syconoid calcareous sponge. Of all particles, only 0.1-microm latex microspheres adhered to the collar microvilli in large numbers, but these were even more numerous on the choanocyte surface. Few large particles (0.5- and 1.0-microm beads and bacteria) contacted the collar microvilli; most were phagocytosed by lamellipodia at the lateral or apical cell surface, and clumps of particles were engulfed by pseudopodial extensions several micrometers from the cell surface. Although extensions of the choanocyte apical surface up to 16 microm long were found, most were 4 microm long, twice the height of the collar microvilli. These observations offer a different view of particle uptake in sponges, and suggest that, at least in syconoid sponges, uptake of particles is less dependent on the strictly sieving function of the collar microvilli.