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This paper presents the study and implementation of a reliable transport protocol for wireless multimedia sensor networks. The work was based on an existing transport protocol for sensor networks, the DTSN ("distributed transport for sensor networks") developed at Inesc within projects IST FP6 UbiSec&Sens, EuroFGI and FP7 EuroNF. For multimedia data transport, some structural changes were made to DTSN, and mechanisms were added in order to provide a differentiated reliability service for packets flowing on the sensor network. The new service allows a multimedia data stream to be divided on several blocks with different reliability. The blocks' size and minimum reliability required are sent to the network by the client to configure and initialize the differentiated reliability service. Lost packets from multimedia blocks with the minimum reliability already assured will not be claimed lost by the receiver, and therefore will not be retransmitted. That will cause fewer retransmissions on the network, throughput improvements and an increase of network lifetime with no significant degradation of the transmitted stream. A packet recovery module was also implemented, based on an additional parity packet that must be sent for each block, allowing the recovery of one lost packet per multimedia block. This module works on top of the transport layer, improving overall efficiency and being completely transparent to it. The new multimedia DTSN protocol was tested in the TOSSIM network simulator. Functional and performance tests were made which have demonstrated the advantages of the proposed solution for multimedia transport in wireless sensor networks.