Cognitive radio (CR) is a novel concept for improving spectrum utilization in wireless communication systems by permitting secondary (unlicensed) users to access those frequency bands which are not currently being used by primary (licensed) users. A CR user has the ability to change its transmit parameters rapidly according to the environment it senses. Orthogonal frequency division multiplexing (OFDM) modulation is a good candidate for CR systems due to its flexibility in allocating resources among secondary users. In this paper, the design of a fast and efficient method for dynamically allocating subcarriers, transmit powers and bits to secondary users in a multiuser (MU) OFDM-based CR system is considered. A memetic algorithm (MA) is proposed and shown to provide an improved performance over previously reported algorithms.