Although nematodes are a well studied group of multicellular organisms, until now the only information on the cellular structure of the male reproductive system of marine nematodes is that on the histology of free-living marine nematode from the order Enoplida. The fine structure of the male reproductive system of the free-living marine nematode Paracyatholaimus pugettensis (Chromadorida: Cyatholaimidae) from the Sea of Japan has been studied using TEM. The testis epithelium has a large distal tip cell similar to that described for representatives of the subclass Rhabditia. The epithelial wall of the testis is differentiated along its length. The proximal part of the epithelial tube consists of relatively large cells bearing numerous surface outgrowths that permeate between the developing spermatocytes. The epithelium in the middle region of the testis is formed from extremely flattened cells. The distal part of the testis – the seminal vesicle – is filled with immature spermatozoa and consists of absorptive cells. The seminal vesicle is followed by the vas deferens. The gonoduct is also differentiated along its length, the first third being formed from synthetically active epithelial cells, the two layers of which form a tiled structure. There is no lumen in the gonoduct and it is probable that, due to the tiled structure, the epithelial cells move apart to create space for the spermatozoa during ejaculation. The posterior two-thirds of the duct is surrounded by muscle cells that create the necessary pressure during ejaculation. The enlarged epithelial cells of the vas deferens show vigorous synthetic activity, which is probably involved in the transformation of immature spermatozoa into mature gametes.