In deuterium NMR spectra of phosphatidylethanolamine bilayers with an extremely high content of saturated fatty acids, each C1 deuteron of the glycerol backbone gave rise to a doublet [Yoshikawa et al., (1988) Biochim. Biophys. Acta 944, 321-328]. This suggests the presence of two backbone conformations, the exchange between which is slow on an NMR time-scale. The origin of the two conformations has been investigated in this work using saturated 1,2-diacyl-sn-glycero-3-phosphoethanolamine specifically deuterated in the glycerol backbone. The results showed that the two conformations originate from different domains, which have different fatty acid compositions. The differential scanning calorimetry of the bilayers suggested that the size of the domain is not large enough to show an independent phase transition. Thus, the formation of microdomains in the phosphatidylethanolamine bilayers has been concluded. Conformational difference in different domains was shown to be restricted to the C1 position of the glycerol backbone. The microdomains of phosphatidylethanolamine were retained even in the presence of other phospholipids.