ABSTRACTMicrofilum lutjanin. g., n. sp. (Microsporida) was found on the gill filaments ofLutjanus fulgens(Teleost) inhabiting the coasts of Senegal. This microsporidium forms xenomas distinguished by the microvilli covering the plasma membrane. At all stages of development individuals have isolated nuclei and are in direct contact with the host cytoplasm. Merogony is binary and sporogony is tetrasporoblastic. the spore (4.75 times 2.60 μm)) is characterized by a manubrium inserted on a laterally offset anchoring disc and extending into a very short, noncoiled polar filament (no longer than 500 nm) in the form of a hook. This type of polar filament has not been described previously in the Microsporida