A T-DNA-tagged population of Arabidopsis was screened for mutations in AtOPT3, which encodes a member of the oligopeptide (OPT) family of peptide transporters, and a recessive mutant allele, opt3, was identified. Phenotypic analysis of opt3 showed that most homozygous embryos were arrested at or before the octant stage of embryo development and that none showed the usual periclinal division leading to the formation of the protoderm. This defective phenotype could be reversed by complementation with the full-length, wild-type AtOPT3 gene. A ß-glucuronidase (GUS) fusion to DNA sequences upstream of the putative AtOPT3 ATG start codon was constructed, and the expression pattern was assayed in transgenic plants. AtOPT3 was expressed in the vascular tissues of seedlings and mature plants as well as in pollen. Consistent with the function of AtOPT3 in embryogenesis, AtOPT3::GUS expression also was detected in developing embryos and in the maternal tissues of seeds. These data suggest a critical role for peptide transport in early embryo development.