We analyse the stellar content of a large number of HII galaxies from the continua and absorption features of their spectra using population synthesis methods, in order to gain information about the star formation histories of these objects. We find that all galaxies of our sample contain an old stellar population (>=1Gyr) that dominates the stellar mass, and in a majority of these we also found evidence for an intermediate-age population >=50Myr apart from the presently bursting, ionizing young generation <=10^7^yr.