— As an optical signal propagates along a lightpath to its destination in wavelength-routed optical networks (WRONs), the quality of service (QoS) will be degraded by transmission impairments such as crosstalk, which is induced by signals traversing the same optical crossconnects, demultiplexers, and fiber segments. Consequently, the signal’s bit error rate at the destination’s receiver might become unacceptably high. Recently, many QoSaware wavelength assignments (WAs) have been proposed [1], [2], [3], [4] that corporate physical layer blocking; these have a high computational complexity compared to traditional WAs because of QoS estimation. In this paper, a new dynamic WA scheme using a wavelength spectrum separation technique is proposed to combat crosstalk occurring in WRONs. Our simulation results show that the new WA scheme has much lower complexity than QoS-aware WA schemes yet has a performance close to the QoS-aware WA schemes.