Thermotropic and barotropic phase behavior of the dinonadecanoylphosphatidylcholine (diC19-PC) bilayer membrane was investigated by use of differential scanning calorimetry and high-pressure light-transmittance technique. The constructed temperature (T)-pressure (P) phase diagram showed the polymorphism of the gel phase (i.e., lamellar gel, ripple gel and interdigitated gel phases) which is one of the common features to symmetric saturated diacyl-PC bilayer membranes. The pre- and main-transition temperatures and the critical interdigitation pressure (CIP) of the diC19-PC bilayer membrane were revealed to be 57.5 °C, 60.6 °C and 37.7 MPa, respectively, which were in good agreement with the values expected from the acyl chain-length dependence of those quantities based on the previous results for bilayer membranes of a homologous series of diacyl-PCs with shorter acyl chains. On the other hand, the enthalpy (ΔH) and volume (ΔV) changes of the main transition for the diC19-PC bilayer membrane were significantly larger than those expected from the acyl chain-length dependence. This may indicate that there are some intrinsic difference in molecular packing between bilayer membranes of diC19- and other diacyl-PCs with shorter acyl chains, and that the diC19-PC bilayer forms a more condensed gel phase than the latter bilayers.