  We introduce the concept of \textit{quasi-stable} ideal in an integral domain
$D$ (a nonzero fractional ideal $I$ of $D$ is quasi-stable if it is flat in its
endomorphism ring $(I \colon I)$) and study properties of domains in which each
nonzero fractional ideal is quasi-stable. We investigate some questions about
flatness that were raised by S. Glaz and W.V. Vasconcelos in their 1977 paper
\cite{GV}.
