Techniques for bundling and ciphering data prior to transmission are described. In an exemplary design, a transmitting entity receives a plurality of service data units (SDUs) from an upper layer, assigns sequential sequence numbers to the SDUs, and bundles the SDUs into a single protocol data unit (PDU). In one design, the transmitting entity generates a single count value based on a sequence number of a designated SDU (e.g., the first SDU) in the PDU and ciphers all SDUs based on the single count value. In another design, the transmitting entity ciphers each segment of at least two SDUs in the PDU based on a count value for that segment. The transmitting entity generates a header with at least one sequence number and possibly length and/or other information for the SDUs. The transmitting entity forms the PDU with the header and the SDUs and passes the PDU to a lower layer.