  We establish the following model-theoretic characterization: profinite
$L$-structures, the cofiltered limits of finite $L$-structures,are retracts of
ultraproducts of finite $L$-structures. As a consequence, any elementary class
of $L$-structures axiomatized by $L$-sentences of the form $\forall \vec{x}
(\psi_{0}(\vec{x}) \ra \psi_{1}(\vec{x}))$, where
$\psi_{0}(\vec{x}),\psi_{1}(\vec{x})$ are existencial-positives $L$-formulas,
is closed under the formation of profinite objects in the category {\bf L-mod},
the category of structures suitable for the language $L$ and $L$-homomorphisms.
