The importance of colonization of subterranean portions of crop plants by bacterial biocontrol agents needs to be defined and investigated. This paper describes the ongoing studies in our laboratories concerning the importance of colonization by Enterobacter cloacae to its suppression of the plant pathogen Pythium ultimum on cucumber. The use of E. cloacae mutants, which are reduced in cucumber spermosphere colonization, and an environmental containment strategy for this biocontrol agent is also discussed.