In a previous paper (Boschma, 1935) I made some remarks concerning the parasites of Pachygrapsus crassipes Randall from Japan, viz., Sacculina confragosa and the specimens mentioned then under the name Sacculina rotundata. The latter parasite, indeed, shows some resemblance to S. rotundata, but a closer examination of its characters shows that it is specifically distinct from this species. It appears to be a new species which is described in the present paper as Sacculina yatsui. Moreover the results of a more or less detailed investigation on Sacculina rotundata are given here to show the differences between the two species. Sacculina yatsui nov. spec. Sacculina rotundata: Boschma 1935. Material examined: Sagami Bay, 1901, Dr. Haberer. Zool. Museum Munich. 2 specimens. Sagami Bay, March 1903, Dr. Haberer. Zool. Museum Munich. 2 specimens. Aburatsuko, Misaki, litoral, August 1929, Prof. A. S. Pearse. Museum Leiden. I specimen1). Misaki, Marine Biol. Sta., 1930, Prof. Naohide Yatsu. Museum Leiden. 5 specimens (holotype and paratypes). Shimoda, Shizuo-ken, sea shore near Marine Biological Laboratory, May 3, 1934, Prof. Yaichiro Okada. Museum Leiden. 3 specimens. All the specimens mentioned above are parasites of Pachygrapsus crassipes Randall. Diagnosis. Male genital organs in the posterior part of the body, outside the visceral mass. Testes more or less globular. Colleteric 1) The statement in my previous paper (Boschma, 1935, p. 152) is erroneous;