We address crystal nucleation and fcc-bcc phase selection in alloys using a multi-phase-field model that relies on Ginzburg-Landau free energies of the liquid-fcc (face centered cubic), liquid-bcc (body centered cubic), and fcc-bcc sub-systems, and determine the properties of the nuclei as a function of composition, temperature and structure. With a realistic choice for the free energy of the fcc-bcc interface, the model predicts well the fcc-bcc phase-selection boundary in the Fe-Ni system. PACS numbers: 64.60. 64.60.My, 82.60.NFreezing of undercooled liquids often starts with the nucleation/formation of metastable (MS) crystalline phases. In agreement with Ostwald's step rule, atomistic simulations imply that the first crystal structure to form is the one, whose free energy is the closest to the free energy of the liquid [1]. In alloys this represents a multiphase multi-component solidification problem. To date, the most efficient method used for addressing such problems is the multi-phase-field theory (MPFT) [2]. It is, however, only as accurate as the free energy functional it relies on. Early versions [2] of the MPFT predicted that the third phase inevitably appears at the interface between two bulk phases, a behavior originating from the specific free energy surface assumed. A recent version of MPFT eliminated the third phase entirely at the interface [3]. This is not always in agreement with real systems: Atomistic simulations for the Lennard-Jones (LJ) system show that although the stable phase is fcc, small nuclei have a bcc structure, and even the larger fcc crystallites have a bcc-like layer at the solid-liquid interface [1,4], results also born out by the classical density functional theory (CDFT) [5]. These findings accord with the theoretical prediction of Alexander and McTague that in simple liquids the formation of bcc structure is preferred [6]. Further simulations for the LJ system imply that varying the pressure at fixed temperature, the bcc/fcc phase ratio can be tuned in small clusters [7]. Since preference for MS phase nucleation is quite general, it is desirable to work out microscopic models that can handle the structural aspects of phase selection during nucleation.In this Letter, we present such a microscopic model for competing fcc and bcc structures. The MPFT is supplemented with a free energy that is based on the GL expansion of the two-phase free energies [8][9][10], and considers thus the structural aspects of multiphase solidification. Our approach is unique in that it combines crystal structure with thermodynamic and interfacial data of real systems. In this respect our MPFT is more flexible than recent CDFT approaches [11], which, in turn, provide a more detailed description of the solid-solid interface. Herein, we apply the GL-free-energy based MPFT to predict phase-selection in the Fe-Ni system.The standard MPFT form of the grand free energy of a binary system relative to the initial liquid is:(1) The differential operator on the right hand side has the required symmetries [2...