We demonstrate the ability to coherently control ultracold atomic Rb collisions using frequencychirped light on the nanosecond time scale. For certain center frequencies of the chirp, the rate of inelastic trap-loss collisions induced by negatively chirped light is dramatically suppressed compared to the case of a positive chirp. We attribute this to a fundamental asymmetry in the system: an excited wavepacket always moves inward on the attractive molecular potential. For a positive chirp, the resonance condition moves outward in time, while for a negative chirp, it moves inward, in the same direction as the excited wavepacket; this allows multiple interactions between the wavepacket and the light, enabling the wavepacket to be returned coherently to the ground state. Classical and quantum calculations support this interpretation.PACS numbers: 32.80. Qk, 32.80.Pj, 34.50.Rk Traditionally, the fields of ultracold physics [1] and short-pulse coherent control [2, 3] have followed independent paths. Besides the obvious incompatibility of sub-picosecond timescales with the motion of slow atoms, cooling typically deals with translational degrees of freedom while coherent control involves internal degrees of freedom. A potentially fruitful collaborative venture is the production of ultracold molecules by coherently-controlled photoassociation of ultracold atoms. Despite many theoretical proposals on this topic [4,5,6,7,8,9,10,11], experiments to date [12,13] have demonstrated coherent control of only the photodestruction of ultracold molecules, not of their collisional formation. Here we describe our use of frequency-chirped light on the nanosecond timescale [14] to coherently control ultracold collisions in Rb. We observe significant differences in the collision rates for the two chirp directions. For the negative chirp, a wavepacket excited to the attractive molecular potential can subsequently be returned coherently to the ground state, thereby reducing the collision rate.Ultracold molecules [15] are of significant current interest due to potential applications in a variety of fields: ultracold chemistry, quantum computing, novel quantum degenerate systems, and tests of fundamental symmetries. One method of ultracold molecule production is photoassociation [16], where two ultracold atoms collide in the presence of laser light and undergo a free-to-bound transition to form an excited molecule. Spontaneous emission can subsequently produce ultracold molecules in the electronic ground state, but typically in a distribution of high vibrational levels. If the entire process could be done coherently, it might be possible to populate a * Present address: