The spermatophore morphology of 13 species of hermit crab from the families Coenobitidae, Diogenidae and Paguridae is described and illustrated, and comparisons are made with existing descriptions to show that spermatophore form, at the light microscope level, can be used to separate three families of the Paguroidea. Spermatophores from members of the family Coenobitidae are robust in nature with large, ovoid-spherical ampullae mounted on short, thick stalks. Members of the family Diogenidae have more fragile spermatophores with small spherical ampullae mounted on long, slender stalks. The spermatophores of members of the family Paguridae are distinctive in possessing large, elongate, ampullae, an accessory ampulla at the base of the main ampulla and a pseudo-stalk analogous with the true stalk of the Coenobitidae and Diogenidae. The occurrence of double-headed spermatophores (two ampullae on a single stalk) is recorded for the first time, in a Dardanus species. The ultrastructure of the lateral ridge, which divides the ampulla of the paguroidean spermatophore into two halves, is described using both scanning and transmission electron microscopy. A simple, branching key for classifying the investigated hermit crabs (from the families Coenobitidae, Diogenidae and Paguridae only) into their respective family, based on the gross morphology of their spermatophore, is presented.