The male gonopores, internal male reproductive apparatus, and spermatophores of the Mediterranean hermit crab Clibanarius erythropus are described, using interference phase microscopy and scanning electron microscopy. A discussion on the relationship between the gonopore morphology and the different kinds of setae accompanying them, and the reproductive biology of these crabs is presented. Each testis merges into a tubular duct made up of four zones: 1) the collecting tubule with free sperm; 2) the proximal zone, where the ampulla of the spermatophore starts to be formed; 3) the medial zone, where the ampulla is completed, the stalk lengthens and the pedestal is formed; 4) the distal zone, where the mature spermatophores are stored. In the mature spermatophore the two halves of the ampulla meet at the distinct lateral ridge. Sizes of the different parts of the spermatophore are given and their morphology described. The importance of the morphology of the paguroid gonopore, male reproductive system, and spermatophore is confirmed for phylogenetic studies within the Paguroidea and Anomura.