Sulfolobus acidocaldarius, strain B12, which harbours a double-stranded DNA species both as a plasmid and in a linear form, which is integrated at a specific site of the chromosome, produces virus-like particles upon u.v. irradiation. These particles contain the same circular DNA and a number of coat proteins and are probably surrounded by a lipid membrane. They are lemon shaped, 100 x 60 nm in size and carry tail structures at one pole. The host cell recovers and remains lysogenic after virus production. Though a large fraction of liberated particles is found attached to structures derived from the cells, neither adsorption nor infection of a number of Sulfolobus isolates has so far been observed.