In this Letter, we report studies of coherent resonances in the course of a single bimolecular collision on the femtosecond timescale. A classical picture is presented with molecular dynamics simulations. The observed persistence of coherence elucidates the origin of the very limited spreading of the wavepacket and the crucial role of selective probing of a limited range of impact parameters. The study of this three-body system (iodine and argon) at room temperature is important for the mechanism of resonances and caging with implications for the condensed phase. (C) 1999 Elsevier Science B.V. All rights reserved.