In this Note we give the asymptotic behaviour for the life-span of classica
l solutions to the Cauchy problem for general quasilinear strictly hyperbol
ic systems with small and decay initial data and prove that the formation o
f singularity is due to the envelope of characteristics of the same kind. (
C) 1999 Academie des Sciences / Editions scientifiques et medicales Elsevie
r SAS.