We present a theoretical analysis of the mode-coupling dynamics of heterogeneous 6- and 7-core multicore fibers (MCFs). Simple analytical expressions for the mode amplitudes are derived, which highlight the effects of dissimilar cores on the coupling characteristics. Although power transfer is confined among the group of identical cores, the coupling within each group is significantly affected by the presence of neighboring dissimilar cores and is dependent on the propagation constants of the eigenmodes of the composite multicore structure. The dissimilar cores can enhance or mitigate the coupling effects by choosing appropriate design parameters. The formulas can serve as a basis for understanding the cross-talk of MCFs for more complicated cases involving longitudinal perturbations. © 2012 IEEE.