SUMMARY: Cultures of Proteus hauseri may consist of one or more of five colonial variants. Population pressure experiments started with one variant eventually yielded all other variants. Y variants form raised non-swarming colonies on a MacConkey-type agar at 37°, but swarm in concentric step-like rings at room temperature on this medium; they swarm in step-like concentric rings on nutrient agar at room temperature and at 37°. This variant is stable under conditions of logarithmic growth and is regarded as the wild type. Variant X is a typical Rough form and does not swarm under any circumstances. In continuous culture it mutates to the Y type at a high rate. The W variant forms colonies very similar to those of the Y variant, but like the X variant does not swarm. It is stable under conditions of continuous culture, but mutates on MacConkey agar to X and Y variants at low rates. On MacConkey agar at 37° the Z type appears as a flat amorphous colony surrounded by a continuous thin film of swarm. It also swarms at room temperature. This variant is replaced by a mixture of Y and W colonial forms in continuous culture experiments. Drop-like colonies, the Tröpfchenform previously described by Weil & Felix (1917) were also encountered. Drop-like colonies are unpredictable in appearance and were not studied in detail. Hanging-drop preparations of 4 hr. old broth cultures at 37° of either W or X colonial variants (which never swarm) reveal many motile forms. Young broth cultures of the other three variants consist of forms which are all actively motile. Lytic phage for a particular strain is active on all its variants except the X or Rough type which is also the only variant to form unstable suspension in saline and in broth. The Y, W and Z variants described here correspond reasonably well to the A, B and C phases of Belyavin (1951). The H, O terminology of Weil & Felix (1917) is not suitable and the nomenclature of Belyavin (1951) may be more satisfactory.