Bimodality of an order parameter distribution is expected as a generic signal of phase transition in finite systems.We recall that the heaviest fragment of partitions observed in nuclear fragmentation was recognized as order parameter thanks to the theory of universal fluctuations.We present analyses of experimental data from Au on Au collisions at 60, 80 and 100 MeV per nucleon showing that bimodality of the heaviest fragment distribution is largely independent of the data selection procedure and of entrance channel effects.Moreover an estimate of the latent heat of the transition is extracted.