This paper reinvestigates the gross changes which follow the inoculation ofProteus mirabilisonto an agar plate. The sequence of development observed by earlier workers was confirmed: namely, the development of a primary colony at the site of inoculation, followed by alternating periods of swarming and consolidation. The paper"s main contribution is the demonstration, by means of time-lapse photography through the phase-contrast microscope, of the cellular changes which occur during the various stages—in particular, the differentiation of short rods into swarmers, the swarming process itself, the subsequent breakdown of the swarmers into progressively shorter units, and the multiplication of these rods. Of special note is the clear demonstration that most of the swarmers are normal viable cells.