The functions of the blind appendages attached to the proximal intestine of many fish, the pyloric ceca, have been disputed. Hence we recorded morphological parameters and nutrient uptake rates in the ceca and intestine of four fish species (rainbow trout, cod, largemouth bass, and striped bass) with various degrees of cecal development (the ceca contribute 70, 69, 42, and 16% of the total postgastric surface area, respectively). Proline and glucose uptake, measured in vitro, is similar in the ceca and proximal intestine. For these two solutes in these four species, and for 10 other solutes (9 amino acids and 1 dipeptide) in trout, the ceca contribute about the same percentage to uptake as to total gut area. Trout ceca and intestine have similar membrane-bound disaccharidase activity. Separate experiments with trout fed either graded glass beads or a radiopaque marker and then X-rayed show that the ceca fill and empty with particles less than 150 microns and over the same time course as the proximal intestine. Thus ceca are an adaptation for increasing intestinal surface area without increasing the length or thickness of the intestine itself. Fish ceca are entirely different from the distally located ceca of birds and mammals, which have fermentation functions.