The visceral pleura of human fetuses aged from 9 to 36 weeks of gestation was studied by means of transmission electron microscopy. The main components of the visceral pleura (mesothelium, basal lamina and submesothelial connective tissue layer) are formed in the fetal period. They develop asynchronously in different pleural areas, and do not reach maturity. Fetal differentiation of the lung pleura can be divided in two stages--early (until 17 gestation week) and late stage--up to birth. The high mesothelial cells appear later than the flat cells, but the first type predominates in the final covering layer during the period investigated. The most significant developmental phenomena of the mesothelium involve membranous differentiation (the microvillous covering, vesicular system and intercellular contacts). The different transport and secretory potentials of the mesothelial cells during the various prenatal periods are discussed. The mode of development of the basal lamina suggests its mesothelial origin. The elastic membrane appears during the late stage of fetal life. The components of the submesothelial connective tissue layer (fibroblasts, collagen and elastic fibres, blood and lymph vessels) undergo several phases of differentiation.