Seasonal variability in the morphology of the digestive tubules was investigated by means of a morphometric procedure in a population of Littorina littorea (Mollusca, Gastropoda) collected from the Abra estuary (Bay of Biscay, Basque Country) at four different seasons September, December, May and July). Some biometric characteristics of the winkles were recorded throughout the study period. Size parameters did not change significantly but high values of flesh condition (wet weight of flesh in mg per wet shell weight in g) were recorded in December and the lowest values in May. The reproductive state was determined using a planimetric procedure to calculate the relative tissue volume of digestive gland, gonad and storage tissue in the visceral mass. The relative volume of the digestive gland was significantly reduced in May. Gonad tissue volumes were maximal in May and minimal in September. Reserve connective tissue reached a maximum value in September. The quantitative structure of the digestive tubules was also investigated. The following five parameters were used to quantify digestive tubule morphology: mean epithelial thickness (MET), mean diverticular radius (MDR), mean luminal radius (MLR), MLR/MET ratio and MET/MDR ratio. The seasonal changes in the tissue composition of the visceral mass do not affect significantly any of these parameters. Thus, reserve storage/consumption and gamete development did not appear to affect the morphology of the digestive tubules in winkles. Reduced MET and MET/MDR values and augmented MLR/MET values were, however, observed at the breeding-spawning period. Such changes resemble a general stress response which might be related to the high metabolic cost of this phase of the reproductive cycle.