Loma boopsi n. sp. was found in the liver, intestine and pyloric caeca of Boops boops inhabiting the coasts of Senegal. Its ultrastructural characteristics were elucidated. All developmental stages had isolated nuclei and occupied the xenoma without any particular stratification. The merogonial stages were in direct contact with the host-cell cytoplasm. The merogonial plasmodium divided by plasmotomy. Sporogony occurred in sporophorous vesicles and the sporogonial plasmodium divided by plasmotomy. Two morphological types of spores were produced in the same sporophorous vesicle: ovoid spores measuring 3.70 × 2.40 µm and elongate, slightly incurvated spores which were 4.80 × 2.40 µm. They had a thick envelope (about 70 nm), a polaroplast with an anterior lamellar part and a posterior vesicular part, an isofilar polar filament displaying 12–14 (ovoid spores) or 16–18 (elongate spores) spiral turns in a single row of coils and a large posterior vacuole