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We report the first efficient detection of individual fluorescent molecules in solution. This constitutes a significant advance in ultrasensitive detection that should have a variety of applications in biological and chemical science. The observation technique, which involves excitation with a repetitively pulsed laser and time-gated discrimination of fluorescence photons, permits single molecules to be detected and counted as they transit the laser beam. Our observations are consistent with Monte Carlo modelling of the method. The development is an essential step in the realization of a proposed high-speed method for sequencing DNA.