The aedeagi of forty-four species belonging to twenty-six genera in the Ceratophyllidae were studied. A description of the aedeagus of each of these species is provided with illustrations of their anatomy. Based on the comparative study of the aedeagal structure, a classification of the Ceratophyllidae is proposed. This family is here considered to contain four subfamilies. Genera with aedeagal characters of uncertain resemblances, with the remainder of the ceratophyllids, are grouped as insertae sedis. These include Smitipylla, Paraceras, Aenigmopsylla, Amphalius, Glaciopsyllus and Macrostylophora. The Tarsopsyllinae includes Tarsopsylla, Myoxopsylla, Orchopeas and Opisodasys. They are grouped based on the short, simple median dorsal lobe which is angled apically. The second subfamily, the Orbpsyllinae contains two Nearctic genera, Oropsylla and Thrassis. These genera are characterized by the distinctive shape and position of the crochets, each with a tongue-like projection pointing posteriorly. Closest to the Oropsyllinae is the Dactylopsyllinae, which is represented by Foxella, Dactylopsylla, and Spicata. The aedeagi of these genera resemble that of the Oropsyllinae in the shape of the crochets and the absence of the fistula. However, they are distinct in having a dorsally expanded endchamber. The Ceratophyllinae can be characterized by aedeagal characters which are also characteristics of the family, From this subfamily, two tribes can be separated based on the structure of the median dorsal lobe. The Ceratophyllini has a bifid median dorsal lobe. Citellophilus, Callopsylla, Malaraeus, Amalaraeus, Amaradix, Dasypsyllus, Ceratophyllus, Aetheca and Jellisonia belong to this group. The Nosopsyllini consists of Eumolpianus, Megabothris, Plusaetis and Nosopsyllus, all with a non-bifid median dorsal lobe. Among the Ceratophyllidae, Nosopsyllus is considered as the most advanced genus.