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Small organophosphorus compounds stimulate Vγ9 Vδ2 T cells if they serve as ligands of butyrophilin 3A1. Because the most potent natural ligand is (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP), which is the last intermediate in bacterial biosynthesis of isoprenoids that is not found in mammalian metabolism, activation of these T cells represents an important component of the immune response to bacterial infections. To identify butyrophilin ligands that may have greater plasma stability, and clinical potential, we have prepared a set of aryl phosphonamidate derivatives (9a–i) of the natural ligand. Testing of these new compounds in assays of T cell response has revealed that this strategy can provide compounds with high potency for expansion of Vγ9 Vδ2 T cells (9f, EC50= 340 pM) and interferon γ production in response to loaded K562 cells (9e, EC50= 62 nM). Importantly, all compounds of this class display extended plasma stability (t1/2> 24 h). These findings increase our understanding of metabolism of butyrophilin ligands and the structure–activity relationships of phosphonamidate prodrugs.