Arthrobacter globiformis was grown in a semi-defined liquid medium containing added solutes to determine the effects of osmotic stress on its reproduction and cell morphology. There was a progressive reduction in the specific growth rate during exponential phase as the concentration of NaCl was increased, although the final yields of the cultures during stationary phase were not affected. Clusters of branching myceloid cells rather than the typical bacillary forms predominated during exponential phase. These myceloids did not undergo complete septation and persisted into stationary phase. Similar responses were observed with potassium sulphate as the exogenous solute but less dramatic morphological effects were found with added polyethylene glycol or sucrose. The myceloids formed in response to osmotic stress could not be disrupted mechanically but were more sensitive than normal cells to lysozyme, particularly during stationary phase. Addition of osmoprotective compounds such as proline, glutamate, glycine betaine, or trehalose to the growth medium did not significantly relieve the effects of osmotic stress on growth rate or morphology. A. simplex also formed myceloid cells during osmotic stress but A. crystallopoietes did not. These results indicate that arthrobacters exhibit characteristic responses to osmotic stress and suggest these bacteria may contain novel osmoprotective compounds.