This paper studies the routing strategy in non-bifurcated traffic grooming in WDM networks. It is to optimally route the specified traffic over a given logical topology to minimize the congestion of the WDM network. We first present the node-arc formulation. To overcome the computational complexity by implementing exact algorithms, we present an ant colony optimization (ACO) metaheuristic. The computational results compared to those of exact algorithms demonstrate that ACO is a computationally efficient and suitable approach for obtaining high-quality routing strategy in non-bifurcated traffic grooming problem in WDM networks.