The magnetic and magnetoresonance properties of a new single-crystal compound Ni-5(TeO3)(4)Cl-2 are studied. Measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature range (5-300 K) make it possible to conclude that Ni-5(TeO3)(4)Cl-2 is a quasi-two-dimensional antiferromagnet with the easy magnetization axis a* directed perpendicular to the crystallographic plane bc and a magnetic ordering temperature T-N approximate to 21 K. Resonance measurements at 4.2 K in wide range of frequencies (25-105 GHz) and magnetic field (up to 200 kOe) permit us to obtain the frequency-field dependence of the AFMR spectrum for a field applied along the easy magnetization axis a*. It is shown that a magnetic field directed along the anti ferromagnetic axis (H parallel to a*) induces a magnetic phase transition of the spin-flop type, which is found to occur at H-sf approximate to 120 kOe. The magnetic resonance experimental data are described qualitatively in the model of a biaxial anti ferro magnet. (c) 2008 American Institute of Physics.