  We consider positional numeration system with negative base, as introduced by
Ito and Sadahiro. In particular, we focus on algebraic properties of negative
bases $-\beta$ for which the corresponding dynamical system is sofic, which
happens, according to Ito and Sadahiro, if and only if the $(-\beta)$-expansion
of $-\frac{\beta}{\beta+1}$ is eventually periodic. We call such numbers
$\beta$ Ito-Sadahiro numbers and we compare their properties with Parry
numbers, occurring in the same context for R\'enyi positive base numeration
system.
