Myxosporeans of the genus Ortholinea have a worldwide distribution and infect organs and tissues of exclusively marine fishes. Here we describe the morphological and molecular characteristics of Ortholinea mullusi sp. nov. parasitizing the urinary bladder and kidney tubules of red mullet Mullus barbatus collected from the coastal zone of Sinop in the Black Sea, Turkey. Polysporic plasmodia with immature spores were either elongate, 37.0 +/- 4.5 SD (30-50) mu m long and 45.0 +/- 3.8 (40-55) mu m wide, or were round, up to 100.0 mu m in diameter. Mature, free spores were spherical in the frontal view and measured 9.3 +/- 0.2 (9.0-9.7) mu m in length, 8.7 +/- 0.3 (8.2-9.3) mu m in width and 7.7 +/- 0.1 (7.5-7.9) mu m in thickness. We observed 2 polar capsules of equal size, which measured 3.1 +/- 0.1 (3.0-3.2) mu m long by 2.5 +/- 0.1 (2.4-2.6) mu m wide, and the tips of the polar capsules were open towards the sutural line. The prevalence of infection by O. mullusi sp. nov. was 24.5%. Phylogenetic analysis based on nuclear small subunit ribosomal DNA (SSU rDNA) clearly suggested O. mullusi to be a new species, clustered within a lineage comprising O. labracis and O auratae. Pairwise nucleotide similarities and DNA distance values between O. mullusi sp. nov. and sister Ortholinea species also supported this suggestion.