The Escherichia coli DNA binding protein FIS is a transcriptional modulator involved in the regulation of many cellular processes, including the activation of rRNA synthesis. High-level overproduction of FIS in early, mid, or late log cultures resulted in growth phase- and media-specific variations in cell growth, rRNA synthesis, and ribosome content. FIS overproduction caused a pronounced increase in rRNA synthesis for late-exponential cultures but a substantial reduction in cell growth and ribosome content. The addition of simple sugars such as glucose or fructose reversed these phenomena, consistent with the functional role of FIS in carbon metabolism.