Existing long-range population projections imply that the timing of the fertility transition has a relatively unimportant effect on long-term population size when compared with the impact of the level at which fertility is assumed eventually to stabilize. However, this note shows that the effect of the timing of fertility decline is a function of the eventual fertility rate: the lower the eventual fertility rate, the greater the effect of the timing of the transition becomes. This finding has important implications for projection methodology, as well as for policies related to the consequences of long-term levels of population size.