The material examined for this note was obtained mostly by the MUSORSTOM 9 Expedition to the Marquesas, French Polynesia, in 1997. Collections were made from the intertidal zone to 260 m deep. Twenty-four species, of which four are new, are listed. The new species are: Portunus paralatibrachium n. sp. which is very close to P. latibrachium (Rathbun, 1906) but which can be easily distinguished by the shapes of the abdomen and male first pleopod; Thalamita difficilis n. sp. which belongs to the admete group and the auauensis-margaritimana subgroup, but can be distinguished from these two species by a series of characters (particularly the shape and positon of the frontal lobes and the male pleopods); Thalamita pseudopelsarti n. sp. which is very close to T. pelsarti Montgomery, 1931, from which it is distinguished by a few but constant characters (shape and position of the frontal lobes, reduction in the size of the penultimate anterolateral tooth of the carapace, spinulation of the basal antennal article, etc.); Thalamita simillima n. sp. which is very similar to T. pseudospinifera Crosnier, 1975 and from which it can be distinguished by an undivided first anterolateral tooth of the carapace, the spinulation and granulation of the chelipeds and the basal antennal article. A new subspecies, Thalamita auauensis dytica n. ssp., is erected for the specimens from the Indian Ocean previously identified as Thalamita auauensis Rathbun, 1906; they are distinguished by the ornamentation of the chelipeds and the form of the sixth abdominal segment. Two species known until now by their holotypes only have been rediscovered: Charybdis rathbuni Leene, 1938, described from Indonesia, and Thalamita multispinosa Stephenson & Rees, 1967, described from Hawaii. Thalamita seurati Nobili, 1906, known only by its holotype, unfortunately a female, is redescribed and illustrated. The possibility that T. wakensis Edmondson, 1925 is a synonym of the latter species is also discussed.
Crustacea, Decapoda, Brachyura, Portunidae, Portunus, Thalamita, French Polynesia, new species redescriptions rare species.