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We propose a new cooperative transmission scheme, Opportunistic Fountain (OF), based on rateless codes for half-duplex relaying. In the proposed scheme, only the most appropriate relay is permitted to forward the message using rateless codes, thus as an orthogonal transmission protocol, our scheme does not need to synchronize among distributed relays. Furthermore, the transmitted information can also be heard and used even by relay nodes that fail to accumulate sufficient mutual information. As a result these nodes can decode the information faster, enlarging the transmitter candidate set (TCS), in which the best relay is selected. We first analyze the simplified one-relay version of the proposed protocol, and present closed-form expressions for the outage probability. The analysis reveals that the proposed scheme can achieve a gain of about 2 dB over the traditional method, which is supported by extensive simulations. We then evaluate the protocol performance for larger network sizes through Monte Carlo simulation, which confirms the benefits of the scheme. I.