The presence of phosphatidic acid phosphohydrolase (PAPase) in human amniotic fluid is demonstrated. The identity of the enzyme was established by the demonstration that the activity of the amniotic fluid enzyme was markedly decreased by the addition of known inhibitors of the enzyme and the pH optimum. The specific activity of PAPase in amniotic fluid increases from 15 nmol of phosphate release per milligram protein per hour at 20–32 weeks of gestation to 82 nmol in pregnancies of 35 weeks gestation and beyond (p < 0.001).