We have examined the pattern of gene expression of mouse cytokeratin endo A and endo B during postimplantational development and in adult organs by Northern blot and in situ hybridization analyses. Both mRNAs localized in the ectoplacental cone, trophoblastic giant cells surrounding the parietal yolk sac, trophoblast cells in placenta, visceral yolk sac, and simple epithelium of the embryo during postimplantational development and in simple or transitional epithelial tissues in adult organs. These results indicate that endo A and endo B are coexpressed and may play some roles in these tissues.