Throughout vertebrate embryogenesis, membrane bound and intracellular protein kinases govern the fundamental decisions necessary for coordinated cell growth and differentiation. Here we have characterized limk, a novel protein kinase with serine threonine substrate specificity which also contains two LIM domains. We used Northern blot and in situ hybridization techniques to determine its pattern of expression in early mouse development. Between 7.5 and 8.5 d.p.c., limk is expressed in three broad domains within the embryo, the neuroectodermal of the prospective forebrain and mid-brain regions, the cardiac mesoderm, and the newly formed definitive endodermal derivatives the foregut and hindgut. By 10.0 d.p.c. limk remains prominently expressed in the ventromedial regions of the developing forebrain and midbrain, with continued expression in the hindgut. In adults limk is expressed most prominently in the brain. Additionally we have shown that limk is most abundantly expressed in the trophoblast giant cells, from 4.5 d.p.c. onwards. Moreover, high levels of limk expression is associated with the overt formation of giant cells from diploid progenitors, suggesting an involvement for limk in the differentiation of this highly specialized extra-embryonic cell type.