Acute bee-paralysis and sacbrood viruses, which multiply in the honey-bee (Apis mellifera L.), have some properties similar to those of the mammalian picornaviruses. Both bee viruses infect their host via the alimentary canal and are neurotropic in adult bees (Bailey & Milne, 1969; Bailey & Fernando, 1972), although they are serologically unrelated (Bailey, Gibbs & Woods, 1964) and differ fundamentally in their ecology and pathology (Bailey, 1965, 1967, 1969). The two viruses also resemble the mammalian picornaviruses in some physico-chemical properties. For example, they are isometric particles 28 nm in diameter, have sedimentation coefficients of 160 S (Bailey, Gibbs & Woods, 1963, 1964) and are resistant to ether. Lee & Furgala (1965) provided chemical evidence that sacbrood virus contains RNA. In this paper we describe other physico-chemical properties of the two viruses and compare these with those of the mammalian picornaviruses.