  An experimental indication of negative heat capacity in excited nuclear
systems is inferred from the event by event study of energy fluctuations in
$Au$ quasi-projectile sources formed in $Au+Au$ collisions at 35 A.MeV. The
excited source configuration is reconstructed through a calorimetric analysis
of its de-excitation products. Fragment partitions show signs of a critical
behavior at about 5 A.MeV excitation energy. In the same energy range the heat
capacity shows a negative branch providing a direct evidence of a first order
liquid gas phase transition.
