Quantitative determinations of disaccharidase enzymes were performed on intestinal mucosa taken from black and white fetuses with a fertilization age of 2 to 7 months. Alpha-disaccharidase activity was detectable at an intrauterine age of 3 months and reached adult levels by the fourth month of intrauterine age. Beta-disaccharidase activity was negligible at 3 months and failed to reach adult levels by the sixth to seventh month of fetal life. Enzyme activity was greatest in the proximal half of the small intestine of the fetus. The oldest fetus in the study may have developed a disaccharidase deficiency secondary to maternal infection.