The aim of this electron microscopic study was to analyze and compare aspects of various histologically distinct seminoma variants including the classical form (11 cases), anaplastic seminoma (1 case) and seminoma with syncytiotrophoblastic giant cells. Sub-microscopic features of these seminoma types are essentially the same; these various seminoma forms can not be electron microscopically distinguished. Although the cytoplasmic structures of the tumor cells varied, the most striking feature was a high glycogen content. Three types of seminoma cells could be distinguished on the basis of the relation between glycogen accumulation and cytoplasmic organelles: 1. tumor cells with plentiful glycogen and scant organelles, 2. tumor cells packed with organelles including RER, mitochondria and Golgi-apparatus but with finely dispersed glycogen, 3. tumor cells with numerous free ribosomes but few organelles and little glycogen. Glycogenosomes are described here for the first time as distinct organelles in seminoma cells, suggesting a disturbed carbohydrate metabolism. Another typical finding was a network of fine filaments measuring about 4 to 6 nm which probably correspond to actin. By interweaving in all directions they produce a felt-like meshwork on the inner-side of the cell membrane. Based on the composition of the seminoma cell cytoplasm with its collection of special organelles including lamellae annulatae, chromatoid bodies and crystalline structures resembling Lubarsch crystals, the tumor cells may be interpreted as neoplastic germ cells related to the so-called atypical germ cells of the testis.