We present the first local, quantitative measurements of ion current filamentation and magnetic field amplification in interpenetrating plasmas, characterizing the dynamics of the ion Weibel instability. The interaction of a pair of laser-generated, counterpropagating, collisionless, supersonic plasma flows is probed using optical Thomson scattering (TS). Analysis of the TS ion-feature revealed anticorrelated modulations in the density of the two ion streams at the spatial scale of the ion skin depth c/ωpi=120  μm, and a correlated modulation in the plasma current. The inferred current profile implies a magnetic field amplitude ∼30±6  T, corresponding to ∼1% of the flow kinetic energy, indicating that magnetic trapping is the dominant saturation mechanism.