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ABSTRACT. The San Pedro-Mezquital River basin in Nayarit, Mexico has the majority of freshwater prawns of the genus Macrobrachium reported for the Mexican Pacific coast. Six species of freshwater shrimp are reported Macrobrachium americanum, M. occidentale, M. digueti, M. michoacanus, M. hobbsi and M. tenellum. The higher richness and diversity of freshwater prawns are found on the coastal plain. The presence of prawns in relation to the altitudinal gradient is discussed. Keywords: freshwater, crustacean, prawns, diversity, coastal plain, Nayarit, Mexico.The crustaceans of the family Palaemonidae include 36 recognized genera that are distributed in sea, brackish and/or fresh water (Holthuis, 1952). In this family, the genus Macrobrachium Bate (1868) is found, comprising about 246 species that inhabit fresh and/or brackish water environments with a wide geographical distribution in tropical and subtropical regions (De Grave & Fransen, 2011). They are distinguished from other palaemonids genera mainly for presenting the second pair of pereiopods chelate, and often in male longer than entire body (Holthius, 1952). These organisms present a behavior called amphidromy (McDowall, 2007), in which adults live in freshwater and migrate downstream near brackish water to spawn, after larval development, juveniles migrate to freshwater environments (Bauer, 2011). In Mexico, 20 species are reported (Villalobos, 1982; VillalobosHiriart et al., 1993;Wicksten & Hendrickx, 2003;Hernández et al., 2007; Alvarez & Villalobos, 2016); however, the discussion on the presence of some shrimp species on both sides (Atlantic-Pacific), or only on one of the slopes, continues among specialists. According to the region they are known as acocil, chacales, camarones, langostinos, acamayas, cauques, moyas, piguas, popotillos o gambas, are an important resource of the artisanal fishing for the people of each region (Hendrickx, 1995). The San Pedro-Mezquital River originates in the state of Durango and crosses the Sierra Madre Occidental as it passes through much of the state of Nayarit until it ends in the south of the Reserva de la Biosfera Marismas Nacionales Nayarit (DOF, 2010). The singularity of this river, is that it does not have dams in its course by Nayarit, condition that few rivers in Mexico present it. So the species move freely along the river and the basin giving great benefits to the biological diversity that sustains this lotic ecosystem. The presence of freshwater prawns in the state of Nayarit has some reports (Villalobos-Hiriart et al., 1993;Pérez-Velázquez et al., 2011); however, the San Pedro-Mezquital River is not registered to date. Therefore, the present study reports for the first time the freshwater prawns of the genus Macrobrachium in several localities of the San Pedro-Mezquital River, Nayarit River, Mexico.In January, February, May 2009 and from July to September of 2012 several localities of the San PedroMezquital River, Nayarit, were visited between 08:00 AM and 18:00 PM (Fig. 1). The sampling sites of higher altit...