A total of 41 externae (35 single and 3 double) of the parasitic sacculind, Sacculina leptodiae (Sacculindae: Rhizocephala) were investigated during this study. They were obtained from 38 individuals (23 males and 15 females) of the xanthid crab, Leptodius exaratus (Xanthidae: Brachyura) collected from the Egyptian coasts of the Red Sea and Gulfs of Suez and Aqaba. The rate of infection represented 5.50% of all populations (691 individuals, 400 males and 291 females) and was slightly higher in males (5.75 %) than in females (5.15 %). The single 35 externae represented 92.11 %, compared with only 3 individuals have double externae represented 7.89 %. The externae of this parasite were extruded on the abdomens of infected crabs, each was attaching with its interna (rootlets) in the internal body cavity of the host via a very short stalk or peduncle. The externae varied in shapes, size and color based on the developmental maturity stages, which comprised immature (1 - ≤ 3.5 mm), early maturing (>3.5- ≤ 5.0 mm), maturing (>5 – 10.4 mm) and spent (flattened shape)stages. The size of externae varied from 1.0 to 10. 4 mm in breadth and averaged 5.19± 2.76 mm in males and 5.63± 3.17 mm in females. The externae have a dense cover of spaced bundles carrying excrescences composed of hard chitin, varied from 15.6 – 47.0 µm, and averaged 32.74± 8.45 µm. The internae comprised network of rootlets of this parasite invaded ovaries, testes, hepatopancreas and all crab's body cavities.