This note gives a new proof of the `operational extensionality' property of 
Abramsky's lazy lambda calculus--namely the coincidence of contextual 
equivalence with a co-inductively defined notion of `applicative 
bisimilarity'.  This purely syntactic result is here proved using a logical 
relation (due to Plotkin) between the syntax and its denotational 
semantics.  The proof exploits a mixed inductive/co-inductive 
characterisation of the logical relation recently discovered by the author.  
(Full version of an invited paper presented at the <i>3rd Workshop on 
Logic, Language, Information and Computation</i> (WoLLIC'96), May 8--10, 
Salvador (Bahia), Brazil, organised by UFPE and UFBA, sponsored by IGPL, 
FoLLI, and ASL.)