The morpho-anatomy of the reproductive tract and of the germ cells has been useful to establish phylogenetic relationships in many insects. However, these elements remain little known in the Coleoptera Passalidae. Here, the male reproductive tract, the spermatogenesis and the sperm structure are described in the South American Spasalus silvarum Kuwert, 1898 (Passalinae, Passalini), a subsocial species living under decaying bark. In this beetle, the two fusiform follicles of each testicle have a central intrafollicular efferent duct and are divided in septa. This organization of the cysts is different from those known in other insects. Each cyst contains up to 128 filiform spermatozoa with relatively small nuclei (29 A mu m in length) in relation to the length of the flagellum (880 A mu m). Among passalids, the general conformation of the male reproductive tract is similar to that described in other American species, but it differs from the African ones in terms of number and shape of testicular follicles. The data provided from the spermatozoa and from their maturation process contribute to the understanding of the reproductive cell biology of Passalidae.