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A new species of Mysidacea, Pseudomysidetes japonicus, is described. The present new species is clearly distinguishable from the other two species of the genus, P. russelli W. M. Tattersall, 1951 and P. cochinensis Panampunnayil, 1977, in the size of the antennal scale, the number of subsegments in the carpopropodus of the 3rd-8th thoracopod endopods, the length of the penis, the shape of the telson, and the number of spines on the endopod of the uropod. P. japonicus represents the first record of this genus in Japan. On the basis of the rudimentary pleopods in both sexes and the length of the penis in the male, Pseudomysidetes is transferred from the tribe Leptomysini to the tribe Heteromysini. An update of the taxonomic key to genera and subgenera of the Heteromysini is suggested.

A new species of Mysidacea of the tribe Leptomysini, Mysidopsis lata, is described. The present new species is clearly distinguishable from the nearest species of the genus, M. surugae Murano, 1970, by the shape of the telson, the antennal scale, the exopod of the uropod, the number of spines on the endopod of the uropod, and the adult body length. M. lata is the third species of this genus known from Japan.