Five new ammonium phosphinates with formula [XC6H4NHC(O)NHP(O)YO]−[H2Y]+ (Y = N(CH3)(CH2C6H5), X = H (IV), CH3 (V), NO2 (VI); Y = NH(CH2C6H5), X = H (VII), NO2 (VIII)) were synthesised by the reaction of N-arylureidophoshoryl dichlorides with N-methylbenzylamine or benzylamine in the presence of an excess amount of the corresponding amine. All new compounds were characterised by NMR and IR spectral data and elemental analysis. Their antimicrobial activity was tested against some Gram-positive and Gram-negative bacteria and fungi. Compounds IV and VIII exhibited moderate activity in vitro against Bacillus subtilis. In addition, compound VIII moderately inhibited Pseudomonas aeruginosa. The crystal structure of benzylmethylammonium(3-phenylureido)(benzylmethylamino)phosphinate (IV) was also determined. This compound crystallises in the orthorhombic system.