The concept of glycolysis optimization, first presented in Waddell (Biochemical Education 25 (1997) 204–205) and Heinrich et al. (Eur. J. Biochem. 243 (1997) 191–201), is reiterated and clarified in response to questions by Christoffersen et al. (Biochemical Education 26 (1998) 290–291). Indeed, increasing the ATP yield above 2ATP/glucose would lead to a decrease in the ATP production rate and a lower flux of material toward lactate.