In this paper, a multimedia cryptosystem is first proposed and then implemented by a field programmable gate array (FPGA). A stream cipher is designed for the cryptosystem based on a coupled map lattice (CML), which can exhibit extremely complex spatiotemporal chaos. To improve the encryption speed, it is digitized and implemented in the FPGA. A user‐friendly interface is designed for users to input data from a PC, to manipulate the cryptosystem to encrypt/decryt and to observe the results. The data communication between a user's PC and the FPGA is realized via an enhance parallel port. The performance of the cryptosystem, i.e. statistical properties, security and speed, are quantitatively analyzed to be satisfactory. Further, the effectiveness of applying the cryptosystem in a text file, an audio file and an image file is verified by investigating the distributions and correlations of a plain‐media (i.e. plain‐text, plain‐audio or plain‐image) and its cipher‐media (i.e. cipher‐text, cipher‐audio or cipher‐image), and the diffusion with respect to a plain‐media. It is shown that the cryptosystem has high security, high speed, low cost and easy‐to‐use property, and can be widely applied for encrypting multimedia. Copyright © 2008 John Wiley & Sons, Ltd.