This study examines the changes of pH in the lumen of the digestive tract during the transition from larvae to juveniles and in adults of the invertebrate-feeder marine flatfish Solea senegalensis. Digestive pH was determined in the different segments of the gut with a pH microelectrode (tip diameter 600 μm). The smaller larvae in which the measurement was possible ranged between 20 and 40 mg wet weight. This size roughly coincides with the apparition of the first gastric glands. The juveniles and adults of this species have a small stomach, long intestine and lack a pyloric caeca. The gastric pH declined slightly with an increase in weight and never decreased below 6.0 irrespective of the age. In older fish the gastrointestinal pH was significantly lower than in younger ones. The values in the anterior part of the intestine were close to those measured in the stomach, but an increase in pH was observed progressing through the medium and posterior intestine. Acid digestion and proteolysis in the stomach seems to be residual in the Senegal sole. The digestion occurs primarily in its long intestine in a slightly alkaline environment. © 2007 Elsevier B.V. All rights reserved.