Supernova 1972e in NGC 5253

New absolute energy distributions of the Type I supernova 1972e in NGC 5253 extending to about 700 days after maximum light have been obtained. A physical model of the expanding envelope, based on the identification of the feature at 6550 A with Hα, is proposed. It consists of a differentially expanding atmosphere, with electron density ranging from 1010 near maximum light to about 107, 340 days later, illuminated by a photosphere with temperature in the range 10,000 K to 7000 K. Under these conditions, the identification of Ca II lambda8600 and H and K, the Na D lines, and the Mg I b lines forming P Cygni lines are quite plausible