Infectious particles with S values of 114 and 132 were isolated from cells infected with bacteriophage phi chi 174. Electron micrographs of the 132S particle revealed a spherical structure with a diameter of about 40 nm. The 114S particle had spikelike projections and a diameter of about 32 nm. The 132S particles could be converted to 114S particles in vitro. However, pulse and pulse-chase experiments indicated no precursor-product relationship between these two particles in vivo.