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Spermatogenesis of Glaridacris catostomi from Catostomus commersoni (Catostomidae) from Albany, New York (USA) was studied by means of TEM, SEM and squashes. Mature testes of G. catostomi contain all consecutive stages of spermatogene-sis; primary spermatogonia are usually situated at the periphery and mature spermatozoa in the centre of testes. The primary spermatogonium divides mitotically, but the two daughter cells, secondary spermatogonia, remain connected with each other by a cytoplasmic bridge. Spermatogenesis in G. catostomi is of a rosette type. Six incomplete, synchronic cytokineses, five mitotic and meiotic divisions occur simultaneously, resulting in a cluster of four tertiary spermatogonia, then eight quaternary spermatogonia, and subsequently sixteen primary spermatocytes are formed. These enlarge, their nuclei move to the periphery and the cluster of cells takes on the form of rosette. After the first meiotic division, a rosette of thirty-two secondary sper-matocytes is formed. The haploid nuclei of these are smaller and the cell membranes near the centre of the rosette become indistinct as the displacement of nuclei toward the periphery continues. The second maturation division results in sixty-four spermatids. During spermiogenesis, their nuclei subsequently elongate, migrate, and are transformed into electron-dense, filiform nuclei of spermatozoa. Each spermatid forms at the surface a so-called "zone of differentiation". From this conical zone arise initially three elongating processes: cytoplasmic extension and two lateral flagella. The spermiogenesis type observed in G. catostomi is characterised by an early abortion of the second axoneme. The remaining single axoneme, forming the sperm flagellum, elongates in parallel with cytoplasmic process accommodating the sperm nucleus; the two parts are initially separated. The migration of the sperm nucleus induces their lateral fusion, which is, however, very superficial; the flagellar axoneme and sperm nucleus are never incorporated completely into a common sperm body as observed in pseudophyllideans or cyclophyllideans. The spermatozoon of G. catostomi consists of two elongate parts: nucleated sperm body with a row of cortical microtubules and flagellum connected by a narrow, longitudinal bridge throughout nearly the entire length. The flagellum con-sists of a single axoneme of the 9 +' 1' Platyhelminthes type. The value of spermiogenesis type and sperm ultrastructure as tax-onomic tools in platyhelminth phylogeny is discussed.