Pre-print, Working paper: This paper explores the relationship between ownership structure and firm value in a transition economy. It distinguishes firms belonging to the sector of innovative technologies and firms from more "traditional" industries. For the latter, the results of the estimation of a simultaneous equations system give support to the hypothesis that strong monitoring and ownership concentration are beneficial for firm performance. They also show that in high-tech firms, higher ownership concentration does not improve firm performance. The sample consists of all non-financial firms listed on the Warsaw Stock Exchange since its creation in 1991.