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The present study was carried out for a period of five months from July 2017 to November 2017 to assess the fish biodiversity applying diversity indices for existing fish species in Basuakhali beel under Terokhada upazilla, Khulna, Bangladesh. During the study, a total of 38 fish species including 34 native and 4 exotic fish species were recorded under 21 families belonging to 10 orders. Cypriniformes (31.58%) was found as the most dominant order followed by Perciformes (18.42%), Siluriformes and Channiformes (10.53%), Synbranchiformes and Decapoda (7.89%), Tetraodontiformes (5.27%), Beloniformes, Cyprinodontiformes and Osteoglossiformes (2.63%) respectively. Almost 57.89% of species were found least concern, 13.16% near threatened, 10.53% vulnerable, 2.63% endangered and 15.79% were found not threatened in the biodiversity of Basuakhali beel. During the study period average Shannon-Weaver diversity index (H') value was highest in Rumer beel (2.38 ±0.10) and lowest in Gojalmari (1.40 ±0.15). Margalef richness index (d) value was highest in Rumer beel (3.69±0.58) and lowest in Abadh (1.55 ±0.31). Pielou’s evenness index (J’) value was highest in Abadh (0.49 ±0.13) and lowest in Kumromari (0.43 ±0.06). Pielou’s index (J´) showed the equal distribution pattern of fish species throughout the sampling area indicating a stable condition. Simpson dominance index (C) value was highest in Rumer beel (0.85 ±0.03) and lowest in Gojalmari (0.64 ±0.06). Eighteen different kinds of fishing gears were identified under 4 major groups including 5 nets (current jal, ber jal, thela jal, veshal jal and punti jal), 6 traps (guni, chari, charo, duor, tubo and polo), 4 hooks and line (koia borsi, dhun borsi, chip borshi and borshi), 3 wounding gears (koach, anta and jupi) in the beel. Besides, an inclusive management and conservation scheme is crying needed for the beel fishery to enrich the fish species diversity of the Basuakhali beel as well as the country.