Significance Block copolymers are prevalent throughout industry and academe due to their self-assembly into well-ordered nanostructures, but only a handful of morphologies are known with the simplest materials built from two chemically distinct blocks. In this article, we report that AB diblock copolymers can also self-assemble into a structure known as the A15 phase. Theory and experiments indicate A15 occurs throughout a substantial region of phase space with suitable differences in the space-filling characteristics of each block. The observed temperature-dependent phase transitions can only be explained using fully fluctuating field-theoretic simulations, which provide evidence that composition fluctuations play a key role in the self-assembly of block copolymers into the larger class of tetrahedrally close-packed sphere phases.