This paper considers the problem of on-line multicast routing in WDM grooming optical mesh networks without wavelength conversion capability. In such networks, provisioning of connection requests with fractional wavelength capacity requirements is achieved by dividing a wavelength into multiple time slots and multiplexing traffic on the wavelength. We present an on-line multicast traffic grooming algorithm for the concerned problem. The objective is to efficiently route multicast requests with sub-wavelength capacity requirements onto high-capacity wavelengths, and balance the load on the links in the network at the same time. To do so, we propose a cost function, which not only encourages grooming new requests onto the wavelengths that are being used by existing traffic, but also performs load balancing by intelligently increasing the cost of using wavelengths on links. The performance results obtained by experiments on a representative sized mesh network show that the proposed algorithm outperforms the other existing algorithms.