This note is about the relationship between two theories of negationas failure – one based on program completion, the other based onstable models, or answer sets. François Fages showed that if a logicprogram satisfies a certain syntactic condition, which is now called‘tightness,’ then its stable models can be characterized as themodels of its completion. We extend the definition of tightness andFages' theorem to programs with nested expressions in the bodies ofrules, and study tight logic programs containing the definition ofthe transitive closure of a predicate.