In this paper we present an algorithm for balancing traffic flows between lightpaths in WDM networks. The load balancing is based on classification of the traffic flows and the use of state information of the links and optical cross connects. Higher priority traffic flows are directed to less loaded lightpaths thus providing them with higher throughput and lower transport delay than is available for lower priority flows. The load levels of the lightpaths are tuned dynamically to support the required quality of service. Additionally, the algorithm ensures that the aggregate load of the incoming data flows does not exceed the overall capacity of the parallel lightpaths.