Dienes phenomenon, when two identical Proteus cultures are inoculated at different points on the same plate of non-inhibitory medium, the resulting swarming of growth coalesce without signs of demarcation.[1] When, however, two different strains of Proteus are inoculated, the spreading films of growth fail to coalesce and remain separated by a narrow easily visible area. The observation of this appearance, the Dienes phenomenon has been used to determine the identity or non-identity of strains in epidemiological studies.[2] [3]