The application of mobile robotic sensor networks has been widely studied in target localization and pursuit. Conventional target tracking methods always require an explicit system observation model of the target positions, which, however, would fail if such model is not available. In this paper, a distributed target localization and pursuit scheme is proposed based on discrete measurements of the energy intensity field produced by mobile targets. The accurate observation model of such field is not available except some critical bounds. By our control strategy, all robots are categorized into two groups: the leaders, responsible for the target pursuit, and the followers, responsible for the formation and connectivity maintenance. The influence of the system parameters on the convergence of leaders to the local maximum points is analyzed. Finally, the proposed scheme is demonstrated by simulation.