Black phosphorus (BP) stands out among two-dimensional(2D) semiconductorsbecause of its high mobility and thickness dependent direct band gap.However, the quasiparticle band structure of ultrathin BP has remainedinaccessible to experiment thus far. Here we use a recently developedlaser-based microfocus angle resolved photoemission (& mu;-ARPES)system to establish the electronic structure of 2-9 layer BPfrom experiment. Our measurements unveil ladders of anisotropic, quantizedsubbands at energies that deviate from the scaling observed in conventionalsemiconductor quantum wells. We quantify the anisotropy of the effectivemasses and determine universal tight-binding parameters, which providean accurate description of the electronic structure for all thicknesses.