The transition between biofilm and planktonic cells has important consequences during infection. As a model system, we have investigated uropathogenic Escherichia coli (UPEC) strain 536, which forms large biofilm aggregates when grown in iron-restricted tissue culture media. The provision of both inorganic and physiological iron to the media induces dispersal. Aggregates do not disperse upon the addition of exogenous iron when cells are pretreated with either rifampicin or chloramphenicol as inhibitors of transcription or translation, respectively. Aggregates stain with the cellulose stain Calcofluor White, can be prevented by the addition of cellulase to the growth media, and aggregates are broken down in the absence of exogenous iron when cellulase is added. An extension of this study to 12 UPEC clinical isolates identified seven that form cellulose aggregates under iron restriction, and that disperse upon the provision of iron. Consequently, we hypothesize that iron restriction stimulates the formation of cellulose aggregates, which disperse as a result of new gene expression in response to the provision of iron.