The Acarids described in the present paper comprised a small collection made by Dr. L. v. d. Hammen of the Leiden Museum during a visit to Biak Island, Netherlands New Guinea in 1953. The mites were found inhabiting the Cladophora socialis and Red Algae on the stones in the intertidal zone near the Royal Netherlands Naval Base on the south coast of the Island, and were sorted out from a large collection of other small arthropods obtained from a large sample of algae by means of a modified Tullgren apparatus. The actual biotope in which they occured is very fully described in Dr. v. d. Hammen's recent paper 1) in which he describes a new genus and species of Oribatid mite from a single specimen. The rest of the Acarids, Dr. v. d. Hammen has very kindly passed on to me for study and I am very grateful for the compliment of being asked to undertake the description of the various species present in the collection. Besides the small Trombidiid Platytrombidium maritimum sp. nov. contained in the collection were three new species of Mesostigmata, Cyrtolaelaps hammeni sp. nov., Leioseius littorale sp. nov., and Periseius littorale gen. et sp. nov., and two specimens of Hyadesia vietsi sp. nov. These five species are described herein. Suborder MESOSTIGMATA G. Canest., 1819 Superfamily PARASITOIDEA Banks, 1915 Family RHODACARIDAE Oudemans, 1902 Rhodacarinae A. C. Oudemans, 1902, Tijdschr. Ent, 45: 48. Rhodacaridae G. O. Evans, 1957, J. Linn. Soc. London, Zool., 43(29): 221. Genus Cyrtolaelaps Berl., 1887 Cyrtolaelaps A. Berlese, 1887, A.M.S., fasc. 44(5); nec Berl., 1892, A.M.S., Meso-