[en] The ultraviolet spectrum of the B8 Ve star HD 50138, recorded during the December 1978-January 1982 period with IUE, is analyzed. The spectrum is found to be crowded by absorption lines, mostly those of singly ionized iron peak elements. In addition there are broad lines essentially due to superionized species (Si IV, C IV) and a P Cygni profile for the Mg II doublet, similar to that seen in the star AB Aur. A few Fe II emission lines are also detected. The slowly expanding envelope surrounding HD 50138 appears to be decelerated outward with a decreasing ionization. The physical state of this envelope seems to suffer strong variations: a double structure was detected for some of the absorption lines in 1978-1979, and is interpreted as due to an outburst. A net increase of the excitation in 1980 is also observed and vanishes in 1982. In order to explain the unusual profiles of the Mg II lines, a possible fluorescence mechanism driven by Ly-beta is discussed. An accurate list of line identification is also provided.