The function of the pyloric caeca of fish has been uncertain since their detailed description in 345 B.C. by Aristotle. He suggested three hypotheses about their function: "to store up the food," "putrify it up," and "concoct it" (i.e., storage, fermentation, and digestion). Our results for trout, cod, largemouth bass, and striped bass support the third but not the first or second of Aristotle's theories. In all four species, the caeca prove to be a major site of sugar, amino acid, and dipeptide uptake, contributing more uptake than the entire remaining alimentary tract in trout and cod. Caecal brush-border membranes contain hydrolytic enzymes. X-ray plates taken at various times after trout had ingested radioopaque marker, and observations of trout fed blue dye plus glass beads of graded sizes, show that caeca fill and empty of food with the same time course as proximal intestine. Thus, whereas the caeca of mammals and birds serve as fermentation chambers, fish caeca are an adaptation to increase gut surface area.