Genetically modified organisms (GMOs) are increasingly used in research and industrial systemsto produce high-value pharmaceuticals, fuels and chemicals(1). Genetic isolation and intrinsic biocontainment would provide essential biosafety measures to secure these closed systems and enable safe applications of GMOs in open systems(2,3), which include bioremediation(4) and probiotics(5). Although safeguards have been designed to control cell growth by essential gene regulation(6), inducible toxin switches(7) and engineered auxotrophies8, these approaches are compromised by cross-feeding of essential metabolites, leaked expression of essential genes, or genetic mutations(9,10). Here we describe the construction of a series of genomically recoded organisms (GROs)(11) whose growth is restricted by the expression of multiple essential genes that depend on exogenously supplied synthetic aminoacids (sAAs). We introduced a Methanocaldococcus jannaschii tRNA: aminoacyl-tRNA synthetase pair into the chromosome of a GRO derived from Escherichia coli that lacks all TAG codons and release factor 1, endowing this organism with the orthogonal translational components to convert TAG into a dedicated sense codon for sAAs. Using multiplex automated genome engineering(12), we introduced in-frame TAG codons into 22 essential genes, linking their expression to the incorporation of synthetic phenylalanine-derived amino acids. Of the 60 sAA-dependent variants isolated, a notable strain harbouring three TAG codons inconserved functional residues(13) of MurG, DnaA and SerS and containing targeted tRNA deletions maintained robust growth and exhibited undetectable escape frequencies upon culturing similar to 10(11) cells on solid media for 7 days or in liquid media for 20 days. This is a significant improvement over existing biocontainment approaches(2,3,6-10). We constructed synthetic auxotrophs dependent on sAAs that were not rescued by cross-feeding in environmental growth assays. These auxotrophic GROs possess alternative genetic codes that impart genetic isolation by impeding horizontal gene transfer(11) and now depend on the use of synthetic biochemical building blocks, advancing orthogonal barriers between engineered organisms and the environment.