A new microsporidian that infects the lizardfish Saurida undosquamis (Richardson, 1848) that are caught in the Arabian Gulf in Saudi Arabia is described here. This parasite invades the skeletal muscle of the abdominal cavity forming white, cyst-like structures containing numerous spores. The prevalence of the infection was 32·1% (135/420). The spores were oval to pyriform in shape and measured approximately 3·3 μm×2·0 μm. The developing spores were found within parasitophorous vacuoles. In mature spores, the polar filament was arranged into 5 coils in a row. Molecular analysis of the rRNA genes, including the ITS region, and phylogenetic analyses using maximum parsimony, maximum likelihood, and Bayesian inference were performed. The ultrastructural characteristics and phylogenetic analyses support the recognition of a new species, herein named Heterosporis saurida n. sp.
