The authors describe the characteristics and application of a 265 nm AlGaN light-emitting diode LED operated at 1 MHz repetition rate, 1.2 ns pulse duration, 1.32 μW average power, 2.3 mW peak power, and ϳ12 nm bandwidth. The LED enables the fluorescence decay of weakly emitting phenylalanine to be measured routinely, even in dilute solution. For pH of 6–9.2, the authors find evidence for a biexponential rather than monoexponential decay, providing direct evidence for the presence of phenylalanine rotamers with a photophysics closer to the other two fluorescent amino acids tryrosine and tryptophan than has previously been reported. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2245441͔