Ichthyc biodiversity of seagrass meadows from the Northwest coast of Cariaco Gulf, Venezuela. Seagrasses are highly productive coastal ecosystems with a high diversity and abundance of fishes, very important to support artisanal fisheries. We analyzed the fish community structure of Thalassia testudinum in the communities of Manzanillo (M) and La Brea (LB), Northwest coast of Cariaco Gulf, Venezuela. Samples were taken monthly (Nov. 2006-Oct. 2007) from each place, using a beach net. A total of 34 810 fishes were captured, grouped into 13 orders, 36 families and 83 species. In both areas (M and LB), the number of species was similar, but a variation in their abundance was found: a total of 55 species and 13 210 organisms for M, and 58 species and 21 600 organisms for LB. The most abundant species and those with the highest biomasses in both areas were: Nicholsina usta, Haemulon boschmae, H. steindachneri, Harengula jaguana, Halichoeres bivittatus and Hemiramphus brasiliensis. The occasional visitors were the most frequent community components with a 59%, the cyclical and permanent residents were represented by the 22% and 19%, respectively. The H'n average for M was of 1.71±0.64bits/ind., while for LB was of 1.95±0.51bits/ind. The diversity values were directly related to the evenness and inversely related to the dominance. The low values of similarity indexes among localities allow us to assert that these fish communities are dissimilar, because of the structure of each Thalassia meadow and their connectivity with other systems. Rev. Biol. Trop. 60 (2): 635-648. Epub 2012 June 01.