The non-enveloped nucleocapsids were normally 58 to 67 nm in diameter and 330 to 350 nm long. The capsid components formed parallel cross-striations (Fig. 5). Thus,the capsid seemed to be con~posedo f rings of sub'units in a stacked series. The thickness of the rings was veryconstant, usually being 20 nm. The rings were perpendicular to the longitudinal axis of the capsid. In terms of virus morphology, WSSV resembles SEMBV (sys-temicectodermal and mesodermal baculovirus) and differs from BMN (baculoviral mid-gut gland necrosis virus) and PmSNPV ( P monodon single nucleocapsid nuclear polyhedrosis virus = MBV) (Sano et al. 1984,Man et al. 1993, Wongteerasupaya et al. 1995). How-ever,the main clinical sign of white spot caused by WSSV was not described in the SEMBV infected shrimp. To date, it is difficult to guess the relation between WSSV and SEMBV.