A versatile chemosensor 1 (E)-2-(((8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)methylene)amino)-1H-benzo[de]isoquinoline-1,3(2H)-dione, based on naphthalimide and julolidine moieties, was developed for highly distinguishable and selective recognition of Group IIIA metal ions (Al3+, Ga3+ and In3+). Sensor 1 exhibited significant 'off-on' fluorescence responses at 488 nm in the presence of Al3+ and at 570 nm in the presence of Ga3+ and In3+. The same emission of Ga3+ and In3+ could be distinguished through different color changes (from colorless to yellow for Ga3+ and no color change for In3+). Binding constants of 1 for Ga3+ and In3+ are the highest reported to date for an organic chemosensor. A 2 : 1 binding mode between 1 with Al3+, Ga3+ and In3+ is proposed based on electrospray ionization mass spectrometry, Job plot analysis, and theoretical calculations.