Dynamic resource allocation is a key technique that can significantly improve the performance of next generation wireless systems under guaranteed QoS to users. Most of the current resource allocation algorithms are, however, limited to unicast traffics. In practice, how to efficiently allocate various resources in multicast wireless systems is not known. In this paper, we shall study dynamic resource allocation for OFDMA-based single-cell multicast systems. Specifically, we shall formulate an optimization problem to maximize the system throughput given a set of available resources (power and sub-carriers). The optimal resource allocation solution is proposed along with a low- complexity algorithm. In two extreme cases, namely, low and high SNR regimes, the low-complexity allocation algorithm is further simplified. Numerical results will show that the system throughput is significantly improved by using our proposed algorithms.