23 human fetal diaphragms were used to study the structural changes of the phreno-oesophageal membrane at the different prenatal ages. 2 human adult diaphragms were used for comparison. This membrane was found to be formed by both the superior and inferior diaphragmatic fasciae. The phreno-oesophageal membrane was mainly formed by the superior fascia at early stages of development and by the inferior at later ages. The collagenous and elastic components of this membrane increased gradually with age. The crural muscle fibres forming the boundary of the oesophageal hiatus at 10 weeks showed gradual regression and was replaced by tendinous fibres. The latter formed secondary insertion in addition to the primary one in the central tendon. The strip of striated muscle, which was occasionally present at the lower end of the oesophagus, could be attributed to failure of recession of the crural muscle bordering the hiatus. The functional significance of the phreno-oesophageal membrane in the process of deglutition and vomiting and for prevention of hiatus hernia is discussed.