  The known results on compartment system's identification requires
observations and measures on some compartments. However, in many situations
some compartments can not be measured, so the minimization of the compartment's
number to be measured is a very important problem to solve. In this work, this
problem had been solved for a class of compartmental systems. It is proved that
all reversible catenary system is identifiable by the measure of one
compartment, the principal compartment.
