The marine snail, Turritella communis, produces two types of spermatozoa, named apyrene and eupyrene. Eupyrene spermatozoa are usually paired, but unpaired ones are involved in fertilization. Movements of these spermatozoa were analyzed using a video camera with a high-speed shutter. The eupyrene spermatozoa usually swim with the head foremost but are able to swim flagellum foremost. A reversal of the direction of their swimming was found to be the result of a change in the direction of flagellar bend propagation, which changed with calcium concentration. Reversal of the direction of bend propagation was accompanied by a reversal of direction of the rotational movement of the spermatozoa around their long axis, suggesting that the bending waves keep the sense of their three-dimensional form. The swimming speed of apyrene spermatozoa in natural seawater was about one-eighth of that of the eupyrene ones and remained almost constant in highly viscous medium.The swimming speed of conjugated eupyrene spermatozoa was the same as that of unpaired spermatozoa over a wide viscosity range (Ͻ3,000 cP). No advantage of swimming by two spermatozoa could be detected in Turritella spermatozoa.