Ultrastructural study of gonia of various species of male crabs shows that these cells belong to two types : primary and secondary gonia; these are discriminated according to their location inside or outside germinative islets, i. e. by the presence or absence of surrounding somatic tissue. The most original feature of the primary spermatogonia concerns the extrusion of gonial material under the shape of lamellar bodies or granules engulfed by the contiguous mesodermic cells; the latter degenerate when they are full of germ cell remnants. Poorly differentiated, the primary gonia have no great variety of cytoplasmic organelles. However two characteristic structures can be recognized in juxtanuclear position : nuage material (electron dense bodies more or less associated with gonia) and chromatoid bodies. Lipid droplets, the significance of which is unknown occur sometimes. Secondary spermatogonia divide synchronously although they are placed side by side without any connection such as cytoplasmic bridges. No ultrastructural modification has been noticed among the two types of gonia in crabs deprived of eyestalks. Mesodermic tissue distributed between primary gonia probably has a double role : inhibition of the onset of gametogenesis (as in Amphipods) and prevention of gonia cytolysis by collecting of altered elements.