The infection of two Helicobacter pylori strains with a phage-containing supernate of the lysogenic H. pylori strain IMMi 290/89 resulted in a lytic cycle and propagation of phage HP1. In negatively-stained preparations, the empty phage heads measured 55-60 nm in diameter and mature heads measured 50 nm. The flexible, striated phage tail was c. 170 nm in length and 9.5 nm in diameter. The phage showed a mean density of 1.40 g/cm3 in sucrose-density gradients and contained double-stranded DNA c. 22,000 bp in length.