The (beta-chloro-, (beta, beta-dichloro-, and (beta, beta, beta-trichloro-alpha-aminoethyl)phosphonic acids have been synthesized and their inhibitory properties on the alanine racemases [EC 5.1.1.1] and the D-Ala:D-Ala ligases [EC 6.3.2.4] from Pseudomonas aeruginosa and Streptococcus faecalis have been evaluated. The monochloro and the dichloro derivatives of Ala-P exhibit a strong inhibition on the racemases of the two species tested but do not behave as suicide substrates. Only the D-Ala:D-Ala ligase of S. faecalis is inhibited by these compounds. The poor antibacterial activity observed with beta-chloro- and beta, beta-dichloro-Ala-P might be enhanced by the peptide-transport strategy.