Single-carrier transmissions with frequency-domain equalization have gained much interest due to their comparable complexity and performance to OFDM, which conversely suffers from a high peak-to-average power ratio. In this paper, we develop a new frequency-domain block turbo equalizer for single-carrier (SC) transmissions over doubly-selective channels. The main feature of the proposed equalizer is its low complexity, which is only linear in the block length. A comparison between SC and OFDM systems with channel coding in doubly-selective channels is also given.