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During a survey of myxosporean parasites of marine fish in the coastal region of Vietnam, a species of the genus Henneguya (Myxosporea: Bivalvulida) was found in the gill of yellowfin seabream Acanthopagrus latus (Perciformes: Sparidae). White and oval cysts, measuring 145-220 μm in diameter, were detected in the gill lamellae of 4 of 15 fish examined (26.7%). Mature myxospores were elongate, with smooth valves, two similar polar capsules, and having the following dimensions: spore body length 9.9 ± 0.5 (8.9-12.5) μm, body width 6.7 ± 0.3 (6.1-7.6) μm, thickness 5.1 ± 0.2 (4.8-5.4) μm, caudal appendage length 10.0 ± 1 (8.3-11.6) μm, and total myxospore length 19.3 ± 1.4 (16.5-21.5) μm. The polar capsules were ovoid, measuring 3.2 ± 0.2 (2.8-3.9) μm long and 1.9 ± 0.2 (1.5-2.3) μm wide. Each polar capsule has a polar filament with 4-5 coils. Histological analysis revealed plasmodia in the connective tissues of the gill lamellae, but inflammation and deformation of the gills were not observed. In the phylogenetic tree reconstructed from the small subunit ribosomal DNA (SSU rDNA), sequences of the Henneguya specimens found in this study form a distinct branch. Morphological characteristics and molecular data identified a new species, namely Henneguya lata n. sp.