Population divergence in early development may lead to hybrid developmental failure and inviability, and may be a factor driving speciation in flowering plants. We investigate the pattern of early and late embryo and endosperm development of seed from Mimulus guttatus and the closely related, serpentine endemic M. nudatus, and compare it to that of hybrid seed derived from reciprocal, interspecific crosses. We address whether disruption in hybrid seed development is the primary source of reproductive isolation between these sympatric taxa. M. guttatus and M. nudatus differ in the pattern and timing of endosperm and embryo development. Some hybrid seed exhibit early disruption of endosperm development and are completely inviable, while other hybrid seed exhibit comparatively normal early development, but show impaired endosperm proliferation. These developmental patterns underlie the phenotypes of mature seed, which are either very small and flat, (indicating little to no endosperm), or shriveled, (indicating reduced endosperm volume). Both phenotypes have low germination success. Hybrid seed inviability forms a potent reproductive barrier between M. guttatus and M. nudatus. We provide a partial developmental mechanism for the hybrid barrier between M. guttatus and M. nudatus. In addition to illustrating the early stages of endosperm and embryo development, we shed light on the extent of developmental variation between closely related species within this important ecological model system.