The quantity of surface membrane internalized during phagocytosis by Chlorohydra digestive cells was estimated for a range of particle types. Challenge with 2 of these particles, freshly isolated symbiotic algae (FIS) and latex spheres (LS), resulted in a greater (2.5 X) quantity of surface membrane interiorized than with heat-treated symbiotic algae (HTS) and free living algae (FA), Chlorella vulgaris. This discriminatory process was investigated further by a scanning electron microscope (SEM) and transmission electron microscope (TEM) comparison of the surface events associated with phagocytosis of each of these 4 particles. Those particles that were avidly phagocytized, FIS and LS, were both enveloped by a tightly fitting extension of digestive-cell surface, and obtained a prominent surface coating after their injection into the gut of Chlorohydra. Phagocytic challenge with FIS resulted, furthermore, in the rapid formation of a dense microvillar cover on digestive-cell surfaces. HTS and FA, on the other hand, were enveloped by a less closely fitting extension of digestive-cell surface, did not obtain a prominent surface coating, and did not induce the formation of microvilli. In addition, SEM revealed that at least 3 morphologically distinct phagocytic modes were utilized by the versatile nutritive phagocyte of Chlorohydra: (I) envelopment by the progressive movement of numerous, overlapping tubular protrusions (microvilli) over the particle (FIS) surface, forming first a network of tubular interlocking members, and finally a continuous but rough enclosing surface; (2) envelopment by a single, smooth-surfaced, funnel-like extension of digestive-cell surface (FIS, LS, HTS, FA); and (3) envelopment by multiple, broad folds, often of unequal size, and with overlapping margins (Artemia particles).