A bacteriophage (Tphi3) which infects the thermophilic bacterium Bacillus stearothermophilus ATCC 8005 was isolated and characterized. Infection of the bacterium by the bacteriophage was carried out at 60 C, the optimal growth temperature of the host. At 60 C, the phage had a latent period of 18 min and a burst size of about 200. The phage was comparatively thermostable in broth. The halflife of Tphi3 was 400 min at 60 C, 120 min at 65 C, 40 min at 70 C, and 12 min at 75 C. The activation energy for the heat inactivation of Tphi3 was 56,000 cal. The buoyant density of Tphi3 in a cesium chloride density gradient was 1.526 g/ml. Electron micrographs of Tphi3 indicate that the phage has a head that is 57 mmu long. The dimensions and shape of the head are compatible with those of a regular icosahedron. Each edge of the head is 29 mmu long. The tail of Tphi3 is 125 mmu long and 10 mmu wide. There are about 30 cross-striations that are spaced at 3.9-mmu intervals along the tail. Under the conditions investigated, Tphi3 adsorbed slowly to the host. Only 2.8% of the phage adsorbed in 10 min at 60 C, the normal incubation temperature that was used. Tphi3 was not infective to four other thermophilic strains or to two mesophilic strains of bacteria.