Understanding the internal structure of cities is an important problem in urban economics. The interaction between major employment locations plays an important role in shaping the city and how households and firms locate. The distribution of employment in urban areas has been studied by several researchers from theoretical and empirical perspectives. The objective of this paper is to study the distribution of employment in the city of Lille in order to identify the most important secondary business districts (SBD). Such identification is an important step in understanding how these centers influence the urban form and how they should be taken into account in transport and urban policies. We do not only consider areas with large employment densities, we also focus on those areas that have an influence on the spatial distribution of employment. The literature review on this question shows the evolution of the concepts and the difficulties in reaching a consensus around the definition of a sub-center. Based on some relevant works, one can define a subcenter as a city with (i) a (relatively) high influence on the global distribution of the density of employment in a metropolitan area and (ii) a significant influence on the density of employment of its neighborhood. The technical difficulty is to isolate the influence of the main business center (the city center), and focus on the effects produced by the multiple business subcenters. We use statistical methods that are suitable for this problem and obtain a set of results confirming the polycentric structure of the city of Lille and showing how the commune of Roubaix (and Tourcoing) plays a particularly important influence. On average, moving away from a business subcenter decreases employment density by about 20%.