We found that colossal variations of resistance in La 0.225 Pr 0.40 Ca 0.375 Mn O 3 in about 100 μ s occur simultaneously with magnetic avalanches that last about 1 ms . These findings are interpreted as macroscopic fingerprints of the percolative phase separation upon application of the external magnetic field. Depending on the fraction of the ferromagnetic phase x , the colossal magnetoresistance jumps appear to be caused either by fluctuations of x , accompanied by fast propagation of the magnetic avalanches through the sample, or by a phase transition from charge ordered antiferromagnetic phase to charge delocalized ferromagnetic phase.