This paper, the fourth of a series dealing with interference organisms in various regions of the United States, represents a summary of reports describing conditions and problems in the south‐central states and the methods used to combat them. Types of algal growth are discussed, along with river plankton, tastes and odors, treatment plant problems, infestations in distribution systems, interference organisms in impoundments, toxic algae, detection of interference organisms, preventive measures, chemical controls, use of activated carbon, and mechanical controls.