Due to the kindness of Mr. Y. Therezien, Ingénieur-hydrobiologiste of the Centre Technique Forestier Tropical at Tananarive, Madagascar, the Rijksmuseum van Natuurlijke Historie received at several occasions freshwater Crustacea from Madagascar. Two previous reports (Holthuis, 1964, 1965) dealt with the crayfishes and the atyid shrimps of these collections. Among the fresh-water Palaemonid prawns a new species of Macrobrachium was found, which is the subject of the present note. It is a great pleasure to dedicate this beautiful species to Mr. Therezien. Macrobrachium therezieni new species River of Maningory, Fénérive district, Tamatave province, eastern Madagascar; 16 February 1961; Y. Therezien, no. 9. — 1 adult male (holotype); Rijksmuseum van Natuurlijke Historie, reg. no. Crust. D. 21234. Andevoranto, sous-préfecture Brickaville, Tamatave province, eastern Madagascar; 22 September 1963; Y. Therezien, no. 37. — 5 males (paratypes); Rijksmuseum van Natuurlijke Historie, reg. no. Crust. D. 21235. Description. — The holotype, an adult male, has the carapace 25 mm long (including the rostrum); its total length is about 60 mm. In the paratypes, all of which are males, the carapace length varies between 25 and 30 mm, the total length between 56 and 63 mm. The rostrum is rather high and straight; it reaches slightly beyond the antennular peduncle, but fails to reach the end of the scaphocerite. The midrib is curved slightly up at the end, but the upper margin of the rostrum is straight or slightly convex. The dorsal margin of the rostrum bears 12 to 15 teeth, the first four of which are placed on the carapace behind the orbit. The first tooth is placed in the anterior third of the carapace; it is separated