Detailed fine structures of Ergasilus sieboldi female from Egypt was studied using both Light microscopy (LM) and Scanning electron microscopy (SEM). The results demonstrated that adult female is cyclopiform with 10 segments measuring 1.4 ± 0.05 mm in length and 0.051 ± 0.04 mm in width. The first five segments formed prosome which consisted of a cephalosome and four free pedigerous somites. The other five body segments formed urosome which exhibited genital double somite, three abdominal somites and fifth pedigerous somite. The first and second antennulary segments were partially fused anteriorly and distinct posteriorly. The genital double somite is wider than long bearing a pair of longitudinal genital aperture parallels the longitudinal axis of the body and ornamented with three transverse bands of spines ventrally. SEM demonstrated that there were a distinct peg seta and minute setae were found locating on the integument of first endopodal segment of antenna.    A large conical seta was detected on the interpodal sternites and marginal spinules on the integument of 1^st exopodal segments of 2^nd swimming leg for the first description. Abdominal somites were ornamented with incomplete row of sharp spinules.