The microorganisms for assimilating oils, fats or an organic waste material, etc. according to the present invention can efficiently decompose waste oils, fats or organic waste materials, etc. contained in a waste liquid and reduce concentrations of such waste oils, fats or organic waste materials, etc. to a level lower than a given standard level. The microorganisms for assimilating waste oils, fats or organic waste materials, etc. to be used for the present invention include strains belonging to genus Sphingomonas, genus Nocardia, genus Microbacterium or genus Bacillus or a mixture of the strains belonging to genera Sphingomonas, Nocardia, Microbacterium and / or Bacillus. The present invention is further directed to a method for treating waste liquids containing waste oils, fats or organic waste materials, etc. by using the microorganisms for assimilating oils, fats or organic waste materials, etc. or to a method for deodorizing offensive odors resulting from such waste liquids.