Fluorescence behaviour of 4-benzoylamido-N-methylphthalimide (1), designed and developed for selective detection of fluoride ions, is reported. 1 displays $F^-$-induced colour change that allows its detection with the naked eye. The $F^-$ specificity of the sensor system is evident from the fact that unlike $F^-$, other halides do not affect the absorption characteristics of 1. Apart from the colorimetric response, the fluorescence output of 1 is also modulated by $F^-$ in a manner that permits ratiometric fluorescence signalling of $F^-$ as well. It is found that the system can detect $F^-$ in the concentration range of $10^-$ 60 𝜇M. The results of the experiments and theoretical calculations unambiguously suggest that the changes of the electronic absorption and fluorescence behaviour of 1, which have been exploited for signalling purpose, are due to $F^-$-induced deprotonation of the 4-amido moiety of the sensor system.