In a number of widely separated places in the tropics, both in the Atlantic and in the Indo-West Pacific, bright red coloured caridean shrimps have been reported from a most peculiar habitat. This habitat is formed by salt water pools which are seemingly shut off from the sea and are often situated at a considerable distance from the shore; these pools, however, must have an underground connection with the sea as their water level falls and rises with the tides. In several instances the pools are in total darkness (in caves) or in semi-darkness. Apart from this habitat and their red colour, the six species of shrimps so far observed in these pools have very little in common: they belong to 5 different genera and represent 3 different families. All of them seem to be restricted to this type of habitat. It is the object of this paper to bring together all the available information on these species so as to draw the attention to this curious phenomenon. In the paragraphs "Material examined" the following abbreviations have been used to indicate the institutions where the material is deposited: I.R.S.M. = Institut de Recherches Scientifiques de Madagascar, Tananarive; R.M.N.H. = Rijksmuseum van Natuurlijke Historie, Leiden; U.S.N.M. = United States National Museum, Washington, D.C. ATYIDAE Halocaridina new genus Definition. — Atyid prawns of small size. Rostrum unarmed. Carapace without supra-orbital, antennal or pterygostomian spines. Telson with two pairs of dorsal spines. Eyes reduced, but with pigment. Carpus of both first and second cheliped deeply excavated anteriorly. Palm of chelipeds distinct.