We developed techniques for combining scanning electron microscopy with conventional measurements to estimate intestinal surface areas in blue tilapiaTilapia aureaand redbelly tilapiaTilapia zilli. These fish have natural diets that are similarly resistant to digestion. The former feeds on detritus and the latter on macrophytes. Intestinal lengths adjusted for total body length were not significantly different between the two species, but redbelly tilapia had a greater intestinal diameter. Intestinal walls of both species consisted of longitudinal folds winding through the length of the intestine. Mean fold width was greater in redbelly tilapia, but fold height did not differ between species. Scanning electron micrographs of cross sections of intestinal walls showed columnar epithelial cells topped by a brush border (i.e., microvilli) and a lamina propria underneath the epithelium in both species. Mean microvillus height was greater in blue tilapia. Microvillus diameter did not differ between species, but both diameter and height decreased distally. Dimensions of folds and microvilli obtained from micrographs were used to develop a technique for estimating surface area that involved expanding the surface area of a simple tube to the surface area of a tube with folds and microvilli. Microvilli accounted for more than 90% of the digestive surface area in both species. Digestive surface area of blue tilapia was 21% greater than that of redbelly tilapia (1,819 cm2versus 1,504 cm2for a fish of 145 mm total length). Blue tilapia's main adaptive features used to increase surface area of the intestine were greater fold development (i.e., convolutions) and taller microvilli. Perhaps these are adaptations to a detritivorous diet.