Midgut epithelial cells of male and female Aedes aegypti, 3 days after emergence, were compared morphometrically. The results of the present investigation concerning the female, are in good agreement with those of a previous study (Hecker et al., 1974), demonstrating that morphometric investigation of midgut epithelia in A. aegypti can successfully be reproduced, and that the mosquito strain used did not show quantitative morphological changes due to laboratory rearing. In males, the cells of the anterior (A) and posterior part (P, 'stomach') of the midgut differ in their quantitative composition. Higher values are found for the microvilli and for the basal labyrinth in the A-part. On the other hand a higher volume density of the mitochondria is present in the P-part. No significant differences are found in the A-part between males and females. Significant differences, however, are present in the P-part. Distinctly more rer in the female stomach can be correlated with the synthesis of enzymes for blood digestion, which are absent in the male. In addition, the more complex functions of the female P-part are also reflected by higher values for other organelles and membrane systems (e.g. mitochondria, basal labyrinth).