  We study extensions of expressive decidable fragments of first-order logic
with circumscription, in particular the two-variable fragment FO$^2$, its
extension C$^2$ with counting quantifiers, and the guarded fragment GF. We
prove that if only unary predicates are minimized (or fixed) during
circumscription, then decidability of logical consequence is preserved. For
FO$^2$ the complexity increases from $\textrm{coNexp}$ to
$\textrm{coNExp}^\textrm{NP}$-complete, for GF it (remarkably!) increases from
$\textrm{2Exp}$ to $\textrm{Tower}$-complete, and for C$^2$ the complexity
remains open. We also consider querying circumscribed knowledge bases whose
ontology is a GF sentence, showing that the problem is decidable for unions of
conjunctive queries, $\textrm{Tower}$-complete in combined complexity, and
elementary in data complexity. Already for atomic queries and ontologies that
are sets of guarded existential rules, however, for every $k \geq 0$ there is
an ontology and query that are $k$-$\textrm{Exp}$-hard in data complexity.
