One hundred three out of 225 (45.8%) of the Red Sea fish Saurida tumbil were infected with microsporidian parasites. The infection was recorded as tumor-like masses (whitish macroscopic cysts) or xenomas often up to 2 cm in diameter and embedded in the peritoneal cavity. Generally, the infection was increased during winter 63.8% (86 out of 135) and fall to 18.9% (17 out of 90) in summer. Light microscopic study revealed that xenomas were encapsulated by a fibrous layer encircling numerous sporophorous vesicles filled with mature spores measuring 1.7 ± 0.6 (1.5-2.7 μm) × 1.5 ± 0.3 μm (1.2-1.8 μm) in size. Ultrastructural microscopic study showed the presence of smooth membranes of the sarcoplasmic reticulum forming a thick, amorphous coat surrounding various developmental stages of the parasite. The various recognizable stages of the parasite were uninuclear, binucleated, and multinucleated meronts followed by detachment of the plasmalemma of the sporont from the sporophorous vesicle producing sporoblasts. Mature spores consist of a spore coat and spore contents. The spore contents consist of the uninucleated sporoplasm and a posterior vacuole located at the posterior end. The polar tube consists of a straight shaft and a coiled region (26-32 coils) arranged in many rows along the inside periphery of the spore. The polaroplast consisted of an anterior region of closely and loosely packed membranes. Molecular analysis based on the small subunit rDNA gene was performed to determine the phylogenetic position of the present species. The percentage identity between this species and a range of other microsporidia predominantly from aquatic hosts demonstrated a high degree of similarity (>92%) with eight Pleistophora species. Comparison of the nucleotide sequences and divergence showed that the sequence of the present microsporidium was most similar to that of Pleistophora anguillarum (99.8% identity) differing in 13 nucleotide positions. So, the present species was recorded and phylogenetically positioned as a new species of Pleistophora.