Summary. Optical and ultraviolet spectroscopic observations of the nucleus of the galaxy NGC5253 are analyzed. This galaxy presents the typical features of an elliptical system at large distances from its center. However, its nucleus is dominated by an emission complex composed by severa) giant H rr regions. The analysis of the optical spectra shows that the metallic abundances in the nucleus of the galaxy are below the solar values. The presence of O stars can be deduced from the numerous absorption Iines present in the UV spectrum. The UV emission lines indicate a high effective temperature for the ionizing star cluster. The comparison of the observations with evolutive models of W(Hp) and the slope of the UV continuum shows that the age of the brightest knot of the nucleus of NGC 5253 is less than three millions years. The exact age depends on the choice of the extinction law, not well known in this type of objects. An LMC like law (consistent with the low metallicity of the object) leads to an age of 2.3 106 yr, and to an IMF similar or slightly flatter than that found by Salpeter for the solar neighbourhood, with an upper mass limit in the range 60 M 0 < M0P < 120 M 0
