The physico-chemical properties of Flock House virus (FHV) were examined. FHV was shown to have a single-stranded RNA genome divided between two species of MW 1.1 and 0.46 million; the genome was infectious. FHV has a single, coat protein of 43,000 MW. The virus sediments at 142S and has a density of 1.351 g/ml in CsCl at pH 7. These properties indicate that FHV is a nodavirus but it is distinguishable from Nodamura virus and black beetle virus by differences in electrophoretic mobility, the size of the coat protein, particle density and stability in CsCl.