Addition of 5-substituted tetrazoles to dialkyl 2-(N-tert-butyl- or N-adamantyl)iminovinylphosphonates proceeds regioselectively to yield novel tetrazole-containing phosphonoacetamidines – 2-[(Ntert- butyl- or N-adamantyl)imino-2-(tetrazolyl)ethyl]phosphonic acid dialkyl esters – with good yields. The reactions occur via nucleophilic attack of the tetrazole N-1 atom at the C=C bond.