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Since June 2012, samples of wild caught white shrimp, Litopenaeus setiferus, from the Gulf of Mexico, Plaquemines and Jefferson Parishes (Louisiana, USA) with clinical signs of microsporidiosis have been delivered to the Louisiana Aquatic Diagnostic Laboratory for identification. Infection was limited predominantly to female gonads and was caused by a microsporidium producing roundish pansporoblasts with eight spores (3.6 × 2.1 μm) and an anisofilar (2–3 + 4–6) polar filament. These features allowed identification of the microsporidium as Agmasoma penaei Sprague, 1950. Agmasoma penaei is known as a microsporidium with world-wide distribution, causing devastating epizootic disease among wild and cultured shrimps. This paper provides molecular and morphological characterisation of A. penaei from the type host and type locality. Comparison of the novel ssrDNA sequence of A. penaei from Louisiana, USA with that of A. penaei from Thailand revealed 95% similarity, which suggests these geographical isolates are two different species. The A. penaei sequences did not show significant homology to any other examined taxon. Phylogenetic reconstructions using the ssrDNA and alpha- and beta-tubulin sequences supported its affiliation with the Clade IV Terresporidia sensu Vossbrink 2005, and its association with parasites of fresh and salt water crustaceans of the genera Artemia , Daphnia and Cyclops . [ABSTRACT FROM AUTHOR]