Abstract: An improved stop-and-wait automatic repeat request(SW ARQ) scheme over underwater acoustic channels is proposed.In the proposed ARQ scheme,each bit in a packet is repeated N times consecutively to increase transmission reliability.The throughput,theoretically derived for the proposed scheme,is optimized with respect to the bit signal to noise ratio(SNR),the packet length,the number of copies transmitted consecutively per bit and the product of the channel transmission rate and propagation time.Compared to basic and modified versions of SW ARQ scheme described in the literatures,the proposed scheme can improve throughput performance of underwater acoustic communications systems markedly.Moreover,with an increase in SNR,the sensitivity of the optimum N to both the packet length and the product of the channel transmission rate and propagation time can greatly be reduced.