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We report on GROND observations of a 40 sec duration (rest-frame) opticalflare from GRB 080129 at redshift 4.349. The rise- and decay time follow apower law with indices +12 and -8, respectively, inconsistent with a reverseshock and a factor 10$^5$ faster than variability caused by ISM interaction.While optical flares have been seen in the past (e.g. GRB 990123, 041219B,060111B and 080319B), for the first time, our observations not only resolve theoptical flare into sub-components, but also provide a spectral energydistribution from the optical to the near-infrared once every minute. The delayof the flare relative to the GRB, its spectral energy distribution as well asthe ratio of pulse widths suggest it to arise from residual collisions in GRBoutflows \cite{liw08}.If this interpretation is correct and can be supported bymore detailed modelling or observation in further GRBs, the delay measurementprovides an independent, determination of the Lorentz factor of the outflow.

A strong optical flare before the rising afterglow of GRB 080129