Abstract The basal infoldings of the cells from the larval caeca, fore- and hind-midgut of the housefly, Musca domestica, may be branched with few apertures to the underlying space (midgut caeca, fore-midgut and anterior hind-midgut) or may be modified into wide channels with many apertures (posterior hind-midgut). The first third of mid-midgut is composed of interstitial and oxyntic cells. Interstitial cells are characterized by basal infoldings modified into long and narrow channels with particles attached to their cytoplasmic side. Oxyntic cells are cup-shaped and display particle studding the cytoplasmic face of their microvillar membranes. The second third of mid-midgut display flat cells without remarkable features, and the last third, cells similar to those in anterior hind-midgut. Anterior hind-midgut and caecal cells display the most remarkable signs of secretory activity. Carbonic anhydrase is most active in the mid-midgut, and fore-midgut, whereas HCO 3 −-ATPase is found only in fore- and hind-midgut cells. The observations suggest that the interstitial cells absorb water and the posterior hind-midgut cells secrete water. This results in a countercurrent flux of fluid, which is responsible for the enzyme recovery from undigested food before it is expelled. Oxyntic cells in mid-midgut are supposed to be responsible for acidifying the food, which is probably neutralized by HCO 3 − pumped from the hind-midgut cells.