Latex beads and wheat germ agglutinin (WGA) were used to examine the movement of membrane components on amoeboid spermatozoa of Caenorhabditis elegans. The behavior of beads attached to the cell revealed continuous, directed movement from the tip of the pseudopod to its base, but no movement on the cell body. Lectin receptors are also cleared from the pseudopod (4). Blocking preexisting lectin receptors with unlabeled WGA followed by pulse-labeling wih fluorescent WGA showed that new lectin receptors are continuously inserted at the tip of the pseudopod. Like latex beads, these new lectin receptors move continuously over the pseudopod surface to the cell body-pseudopod junction where they are probably internalized. Mutants altering the rate of membrane flow, and eliminating its topographical asymmetry, have been identified. Together with the observation that fluorescent phospholipids are cleared from the pseudopod of developing spermatozoa at the same rate as lectin receptors (25), these results show that there is bulk membrane flow over the pseudopod with assembly at the tip and apparent disassembly at the base. There are no vesicles visible at either the pseudopodial tip or base, so these spermatozoa must have a novel mechanism for insertion and uptake of membrane components. This membrane flow could provide the forward propulsion of spermatozoa attached to a substrate by their pseudopods.