Fourteen new species of dicyemid mesozoans, included within four genera, are described from six shallow-water species of Japanese cephalopods : Dicyema colurum sp. nov. and Dicyema erythrum sp. nov. from Octopus fangsiao ; Dicyema dolichocephalum sp. nov. and Dicyema sphyrocephalum sp. nov. from Octopus minor, Dicyemennea gyrinodes sp. nov., Dicyemennea ophioides sp. nov., and Dicyemennea trochocephalum sp. nov. from Octopus hongkongensis ; Dicyema lycidoeceum sp. nov. and Pseudicyema nakaoi sp. nov. from Sepia lycidas ; Dicyema hadrum sp. nov., Dicyema rhadinum sp. nov., Dicyemennea mastigoides sp. nov., Dicyemennea minabense sp. nov., and Pseudicyema nakaoi sp. nov. from Sepia esculenta ; Dicyemodeca anthinocephalum sp. nov. from Octopus dofleini. The status of Pseudicyema and Dicyemodeca is briefly discussed and the validity of these two genera is reaffirmed. The majority of the dicyemid species examined was found to be host-specific. In a few instances the same species of dicyemid was detected in two different cephalopod host species belonging to the same genus. Host specificity appears to be characteristic of dicyemid-cephalopod relationships. The largest cephalopod host species, namely Octopus dofleini, Octopus hongkongensis, and Sepia esculenta, harbored dicyemid species with the longest or largest vermiform stages. Two new types of cells are described from the infusoriform embryos of Dicyemennea gyrinodes and Dicyemodeca anthinocephalum. Three additional cell numbers, namely, 41, 42, and 43, are recognized from the infusoriform embryos of Dicyemennea gyrinodes, Dicyemodeca anthinocephalum, and Dicyema dolichocephalum, respectively. The cell types and morphology of infusoriform embryos are briefly discussed.