Myxobolus muelleri (Myxosporea: Myxobolidae), a myxosporidian parasite, was found for the first time infecting gills of flathead grey mullet (Mugil cephalus Linnaeus, 1758) from the Samsun coastal waters of Turkey. The shapes of the mature spores were regularly oval. Some spores somewhat tapered anterior end, others completely round. Spores ranged from 8.3 (7.2-9.0) mu m in length, 7 (6.4-7.4) mu m widths and 4.7 (4.5-4.9) mu m in thickness. Two polar capsules were pyriform in shape, equal in size, 3 (2.5-3.5) mu m long and 1.8 (1.5-2.5) mu m wide. Polar capsules were usually extended half of spore cavity. There was a large intercapsular appendix between the anterior ends of the polar capsules. There were up to 6-8 sutural marking along the sutural edge. No iodinophilous vacuole discernible in the sporoplasm of the spores.