In this paper, we investigate single carrier frequency-domain equalization (SC-FDE) for distributed spacetime block codes (D-STBC) in a relay-assisted transmission scenario over frequency-selective fading channels. We assume the special case of a single-relay where the source-to-relay (S → R), relay-to-destination (R → D), and source-to-destination (S → D) links experience possibly different channel delay spreads. Assuming perfect power control between R → D and S → D links and high signal-to-noise ratio for all underlying links, our performance analysis demonstrates that SC-FDE for D-STBC is able to achieve a maximum diversity order of min(L1, L 3) +L2+2 where L1, L2, and L 3are the channel memory lengths for S → R, S → D, and R → D links, respectively. This illustrates that the smaller of the multipath diversity orders experienced in S → A and R → D links becomes the performance bottleneck for the relaying path. For the special case of a non-fading relaying path where line-of-sight propagation is possible in either one of these underlying links, we demonstrate that the maximum diversity orders of L1+L2+2 and L3+L2+2 are achievable assuming non-fading R → D and S → R links, respectively. An extensive Monte Carlo simulation study is presented to corroborate the analytical results and to provide detailed performance comparisons among the competing schemes.