This study considers the routingand wavelength assignment problem (RWA) in optical wavelength-division-multiplexed networks. The focus is dynamic traffic, in which the number of wavelengths per fiber is fixed. We minimize connection blocking usingan ant-colony- optimization (ACO) algorithm that quantifies the importance of combining path-leng th and congestion information in making routing decisions to minimize total network connection blocking. The ACO algorithm achieves lower blockingrates than an exhaustive search over all available wavelengths for the shortest path.