A cytoplasmic dsRNA virus, rotifer birnavirus (RBV), has recently been isolated from the rotifer Brachionus plicatilis and is associated with a high mortality rate. Histologically, the viral lesions consist of characteristic inclusions, particularly amorphous dense bodies containing occluded particles. Purified virions are about 59 nm in diameter, single-shelled and display four capsomers per edge. The purified virions have a buoyant density of 1.290 (full particles) and 1.250 (empty particles) in CsCI gradients. Four major structural polypeptides of Mrs 60K, 52K, 33K and 27K were detected by SDS-PAGE. The genome is composed of two linear segments of dsRNA with Mrs of 2.45 x 106 and 2.31 × 106; additionally, small circular ssRNA molecules were detected by electrophoresis in overloaded agarose gels, but their significance is currently unknown. Except for this last feature and the structural instability of purified virions under freeze storage, all the other biochemical and biophysical characters indicate that RBV is a member of the Birnaviridae family with, for the moment, a unique position in this group.