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Fish diversity in reservoirs is one of the most important food resources for people in villages. Huai Kao reservoir, located in the Na Chauk district of Maha Sarakham province in northeastern Thailand, is a main food source, especially for fish, for villagers in that area. This research aims to study the species diversity and distribution of fish species in Huai Kao reservoir. Six stations were assigned to collect fish samples from January to November, 2022. The samples were collected using a gang of mesh size gillnets with sizes of 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0 centimeters, and then their composition, frequency of occurrence, fish numbers and fish diversity were analyzed. A total of 3691 individuals belonging to 14 families containing 22 species were discovered, with the Cyprinidae family being the most diverse, accounting for 8 species (36.36%), dominated by Hampala dispar (28.72%). The species with the highest frequency of occurrence (100%) were Anabas testudineus, Oxyeleotris marmorata, Cyclocheilichthys armatus, Labiobarbus lineata, Hampala dispar, Notopterus notopterus, and Monotrete fangi. The highest number of individuals was found in Winter, January, and Station 3, with 1881, 579, and 804 individuals, respectively. The average value of the Shannon diversity index (H), Species Richness index (S) and Evenness index (E) was 1.50, 1.74, 0.75 respectively.