Digestive flexibility is a widespread phenomenon among animals, and the congruence between empirical data and optimal digestion models strongly supports the idea that it has evolved by natural selection. However, current understanding of the evolution of this amazing flexibility is far from being comprehensive. Evidence from vertebrate tetrapods suggests that there are two major mechanisms for intestinal down-regulation during fasting periods: a decrease in the number of enterocytes in the mucosal epithelium in endothermic species, and a transitional epithelium in concert with a marked hypotrophy of enterocytes in ectothermic species. Here, we analyze the intestinal changes, at the morphological and histological levels, occurring after 9 and 16 days of fasting in a small characid fish species (Hyphessobrycon luetkenii). We found that short-term fasting was correlated with a marked down-regulation of gut size (i.e., caeca and intestine dry mass fall to a 42.3%, while intestinal length was reduced to a 73.9% of the feeding values) and that these changes were accompanied by a shift in intestinal epithelial organization from a simple columnar to pseudostratified one. This result, in conjunction with data on changes in enterocyte turnover rates during fasting in other fish species, suggests that gut regulation at both levels, cell renewal rate and epithelia configuration, is the basal condition to all tetrapods. More data, especially in some key taxonomic groups (e.g., fish that follow an endothermic strategy), will be needed in order to reach a clear understanding of digestive flexibility evolution. J. Morphol., 2011. © 2011 Wiley Periodicals, Inc.