Testes from the allied rock wallaby(Petrogale assimilus) have been examined by the electron microscopy in thin sections in order to examine spermiogenesis and structure of spermatozoa. The spermiogenesis can be divided into nine stages: early Golgi, late Golgi, collapsing, nuclear protrusion, condensation and flattening, nuclear shaping, rotation, nuclear ring contraction, and maturation. The acrosome has been abruptly formed following the collapse of expanded acrosomal vesicle without the cap stage described in the eutheria. The flatly condensed nucleus rotates obliquely to the axis of the axial filament complex and the folded acrosome covers the anterior third of the dorsal nuclear surface forming a wide subacrosormal space as the nuclear ring has contracted. The Sertoli cell reaction and spur are prominent during the nuclear protrusion and rotation stages. A mature spermatozoon has S-shape head which has an extended part reaching to the vicinity of the middle piece.