Abstract: Two fluorescent functionalised benzo[a]phenoxazinium chlorides were efficiently synthesised by condensation of 5-ethylamino-4-methyl-2-nitrosophenol hydrochloride with the corresponding N-substituted-naphthylamines. In order to evaluate their applicability as covalent probes for biomolecules, these heterocycles were coupled at the C-terminus of N-protected-L-valine through an ester or amide linkage. Studies of their photophysical properties revealed that all compounds absorbed and emitted at longer wavelengths with high fluorescent quantum yields.