Escherichia coli 15T-R1, a temperature-dependent cell division mutant, grows into filaments of various lengths (200 to 500 microgram) at 24 degrees C, but divides essentially normally at 37 degrees C. When grown to late-exponential phase at the restrictive temperature, the elongated cells showed discrete areas of increased density at polar regions and other sites in the cytoplasm, when viewed by phase contrast microscopy. Electron microscopy of preparations specifically stained for polysaccharide revealed clusters of granules with a similar distribution pattern to that of the dense areas seen by phase contrast microscopy. The granules were susceptible to alpha-amylase digestion, and chemical analysis of the extracted and purified polysaccharide showed that it consisted of polyglucose, including glycogen. At 24 degrees C the R1 cells contained about twice as much polyglucose and four times as much glucogen as at 37 degrees C.