Spermatogenesis in Bursaphelenchus mucronatus, described using TEM, is similar to that of the 'rhabditid' nematodes. The development includes formation of complexes of fibrous bodies (FB) and membranous organelles (MO) which appear in spermatocytes; the complexes dissociate in the spermatids; the immature sperm contains separate FB and MO and transformation continues only after activation in the female gonoduct. The spermatheca contains mature spermatozoa as bipolar cells subdivided into one large pseudopod and a main cell body containing a nucleus without a nuclear envelope, numerous mitochondria and peripheral membranous organelles opening to the exterior via pores. Pale reticulate bodies appearing in the B. mucronatus spermatids have no analogies in other nematode spermatogeneous cells. An unusual feature of B. mucronatus spermatozoa is the presence of a very large knob-like projection on each MO marking the pole which joins to the sperm plasmalemma to form a specific pore during in utero spermatozoon activation. The spermatogenesis of B. mucronatus resembles that of Aphelenchoides blastophthorus, although transparent vesicles in spermatids and spermatozoa, filopodia with microtubule-like fibres of immature spermatozoa, eccentric nucleus and multiple pseudopods of the mature spermatozoa distinguish spermatogenesis of the latter from the former. Spermatogenesis includes distinct cytomorphological features that may possibly be used to separate the Bursaphelenchus species and trace their phylogenetic relations.