A plant embryo consists of an embryonic axis, which eventually grows into the adult body, and one or two nutritive structures, the cotyledons. In the grasses embryo morphogenesis can be divided into three periods: during the first the embryo is regionalized into an embryo proper and suspensor, during the second the embryonic axis is established, and during the third vegetative structures are elaborated. Maize, with its well-characterized embryo-genesis, powerful genetics, and transposon tagging stocks, offers an attractive system for mutational analysis of these events. We have isolated 51 embryo-specific (emb) mutations from active Robertson's Mutator maize stocks. These are single-gene recessive lethals that represent at least 45 independent mutation events. Each of the 25 mutations was located to a chromosome arm using a B-A translocation set that uncovers approximately 40% of the genome; the same test failed to locate 20 others. The embryo phenotype of 27 mutations was characterized by examining mature mutant embryos in fresh dissection: the various emb mutations differ in phenotype and each is consistent in its expression. All 27 mutations result in retarded embryos that are morphologically abnormal. Nine mutants are blocked during the first period; 10 mutants are blocked during the second period; and eight mutants are blocked during the third period. Based on both the genetic and developmental data, it is likely that there are many loci that can mutate to give the emb phenotype and that these genes are crucial to the morphogenesis of the embryo.