The cephalosomic appendages, including the rostrum, are redescribed in the bathypelagic harpacticoid Aegisthus mucronatus Giesbrecht, 1891 (Aegisthidae). Profound sexual dimorphism was found in all cephalosomic appendages. For the first time reference is made to the presence of a mandible in the male and figures are given for the male maxillula. Distinct sexual differences are also observed in the structure of the labrum and the labium. The structure and ornamentation of the male antennula clearly shows that the Aegisthidae belong to the monophyletic group Cervinioidea, including also the Rotundiclipeidae and Cerviniidae. Brief comments are made on the nature of the first pedigerous somite, the structure of the anal somite and of the caudal rami. The occurrence of sexual dimorphism in postantennular cephalosomic appendages of harpacticoids is reviewed.