Four Rhizobium meliloti bacteriophages belonging to the Myoviridae family of tailed phages were studied. Burst sizes (50-100 virulent particles per infected cell), adsorption rates (2.6-4.1 x 10(-9) ml/min), and latent periods (2-4 h) made this group heterogeneous. However, these characteristics indicate an important infection ability compared with other rhizobiophages. In vivo morphogenesis of phage CM1, studied by electron microscopy, seems to have steps similar to those of some other tailed phages such as coliphages.