Aims Effects of sublethal levels of cinnamaldehyde (CIN) on the viability and morphology of Escherichia coli O157:H7 and E. coli 8 WT were investigated at 6 and 37 degrees C. Methods and Results The minimum inhibitory concentration of CIN against E. coli O157:H7 and E. coli 8WT was 400 mg l-1. At 37 degrees C and =300 mg l-1, CIN delayed the multiplication of both strains, causing a =5 and =13 h lag, respectively. Delayed multiplication at =300 mg l-1 was partly due to cell elongation and injury as determined by LIVE/DEAD viability, CTC vitality and bis-(1,3-dibutylbarbituric acid) trimethine oxonol staining. The greatest extent of cell elongation (87%) and greatest mean length (6.4 mu m) occurred with E. coli O157:H7 at 2-h exposure to 200 mg l-1 CIN. After initial delays in multiplication, both E. coli O157:H7 and E. coli 8WT returned to exponential growth and normal morphology before reaching the stationary phase. In contrast at 6 degrees C, CIN at =100 mg l-1 prevented cell elongation which occurred in untreated control cells. Treatment with 200 or 300 mg l-1 CIN at 6 degrees C was lethal to both E. coli strains. At 300 mg l-1, CIN caused a =5 log CFU ml-1 reduction at =3 days and completely inactivated both of these organisms, causing =7 log CFU ml-1 reduction at 7 days. Conclusion Sublethal levels of CIN at 37 degrees C delayed the multiplication of E. coli cells by causing transient cell elongation, but at 6 degrees C =200 mg l-1 CIN was lethal to E. coli. Significance and Impact of the Study Inhibition of cold-induced cell elongation and the enhanced lethal effect of CIN at 6 degrees C against E. coli O157:H7 suggest that CIN may be useful for control of this pathogen at refrigeration temperatures.