Out of a series of eight new phosphonate nucleosides with an L-threose and an L-2-deoxythreose sugar moiety, two new compounds were identified (PMDTA and PMDTT) that showed potent anti-HIV-1 (HIV-2) activity [EC50 = 2.53 mu M (PMDTA) and 6.59 mu M (PMDTT)], while no cytoxicity was observed at the highest concentration tested [CC50 > 316 mu M (PMDTA) and > 343 mu M (PMDTT)]. The kinetics of incorporation of PMDTA into DNA (using the diphosphate of PMDTA as substrate and HIV-1 reverse transcriptase as catalyst) was similar to the kinetics observed for dATP, while the diphosphate of PMDTA was a very poor substrate for DNA polymerase alpha. The incorporated PMDTA fits very well in the active site pocket of HIV-1 reverse transcriptase.