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High-resolution spectra have been obtained with the long-wavelength camera of the International Ultraviolet Explorer for five Cepheids at a number of phases each. These data were used to study the profiles of the Mg II h and k lines near 2900 Å. An emission feature often appears at the centers of the two lines. There are also central absorption features due to circumstellar or interstellar gas. They are often multiple and may or may not divide the emission into two peaks. The velocities of the circumstellar absorption components imply that they originate in a region which is at least several tenths of a stellar radius in extent. The strength of the emission rapidly increases to a maximum during rising light. It then tends to decline over most of the rest of the cycle, although there are instances of secondary maxima. The mean flux of the emission is lower than for nonvariable stars of similar temperature and luminosity, but at maximum strength the emission is appropriate to the instantaneous temperature.