1. Iso-agglutination occurs in the black chiton, Katharina tunicata. This is the first report of unmistakable iso-agglutination of spermatozoa of a mollusc; also of iso-agglutination of spermatozoa which are inactive in sea-water.2. The iso-agglutinated masses of spermatozoa of K. tunicata differ from those of sea-urchins and of Nereis. In the former the agglutinating masses resemble three-dimensional pinwheels.In these the spermatozoa are so oriented that the center consists of a nucleus of sperm heads with tails radiating outward, and the periphery is composed of a dense zone of sperm heads with tails radiating inward.In sea-urchins and in Nereis the iso-agglutinating masses resemble whirling swarms in which the spermatozoa either are not oriented or are oriented with their heads forming the central nucleus of the swarm. However, a pinwheel type of iso-agglutination can be obtained with stale Arbacia spermatozoa. The motility of the latter is decidedly subnormal. It is possible that the variation in shape of agglutinated masses may be correlated with the degree of motility of the spermatozoa which are involved.3. Unmistakable iso-agglutination can not be detected in any of the following: Asterias ochracea, Asterina, Asterias forbesii, Lepasterias æqualis, Cumingia, Ishnochiton magdalenensis, Mopalia muscosa, Cryptochiton, and Abalone.4. Two types of hetero-agglutination are here reported: toxic "mass coagulation," similar to that described by Lillie (1913) for Nereis and by Just (1919) for Echinarachinius, and a pinwheel type in Katharina tunicata, which bears a startling resemblance to iso-agglutination in the same species.5. Hetero-agglutination occurs between spermatozoa of K. tunicata and solutions of cytolyzed spermatozoa or of blood of either S. purpuratus or of S. franciscanus. The reciprocal also occurs: hetero-agglutination of spermatozoa of either S. purpuratus or of S. franciscanus by the blood of K. tunicata.6. Hetero-agglutination of spermatozoa of Ishnochiton magdalenensis may be produced by the blood of either S. purpuratus or of S. franciscanus. The reciprocal may also be produced: hetero-agglutination of spermatozoa of either S. purpuratus or of S. franciscanus by blood of Ishnochiton magdalenensis.