A new myxosporean, Leptotheca sparidarum n. sp., is described from the trunk kidney of two sparid fish, Sparus aurata and Dentex dentex, in several culture facilities from the Western Mediterranean coasts. It is distinguished from all the previously described species by spore morphometrics. Spores with two equal polar capsules and one binucleated sporoplasm. Spore measurements from D. dentex were 5–7.1 μ long × 8.8–12.3 μ thick × 5.88–6.18 μ wide; polar capsules 2.6–3.5 μ in diam. Spore measurements from S. aurata were 5.1–8.24 μ long × 9.41–11.76 μ thick; polar capsules 2.4–3.2 μ in diam. No significant differences were detected between either host. Prevalence could reach 21.4% in D. dentex and 19% in some stock of S. aurata. Renal tubules were the typical site of infection, which was also found in ureters and glomeruli, but seldom in the epiepithelial position of the gut. Spores were formed in disporous sporoblasts, and spore maturation seemed to proceed from the host epithelium towards the lumen. Trophozoites were attached to the host epithelial cells of renal tubules by pseudopodial-like projections, which were inserted into gaps between epithelial cells. Also, cell junctions were observed between primary cells and between neighboring spores.