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We have produced amplitude-modulated near-ultraviolet light, centered at 390 nm, using an inexpensive, commercially available blue light-emitting diode (LED). The LED was amplitude modulated with the +13 dBm ac output from a frequency generator while biased through a bias tee with 60 mA of dc current. The LED produced 45 to 54 μW of UV light over the modulation bandwidth of 0.01 to 200 MHz, when measured after optical filters to remove the residual blue output. Since the filter attenuated the UV output about 3 dB, more than 100 μW of UV light was initially produced. Modulated UV light was available to approximately 200 MHz, with a -3 dB point of 31 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetimes of standard fluorophores (9-cyanoanthracene and green fluorescent protein) were measured in the frequency domain using the phase-modulation technique, producing lifetimes that closely agree with those reported in the literature, confirming that the UV-emitting blue LED is practical for spectroscopic and sensor applications. When compared to a laser modulated with a Pockels cell, the LED was smaller, less expensive, required less power, generated less heat, and required less alignment. The ability to modulate the LED at high frequencies, along with the UV output, allows its use as an inexpensive UV light source in fluorescence lifetime optical sensors and even frequency-domain fluorometers. © 1997 American Institute of Physics.