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GaAs/AlGaAs single quantum well samples have been investigated by photoluminescence spectroscopy using a probe size of about a micron at low temperature. Thin quantum wells fabricated by molecular beam epitaxy with growth interruptions at the interfaces reveal intense photoluminescence lines with spectral widths below 0.1 meV at the low-energy side of the main luminescence. Mapping the quantum well by scanning the probe shows local emission of these sharp lines, which change in number and in energy with lateral probe position. The local source of the sharp lines, as well as their temperature and saturation behavior shows that they are caused by single excitons localized at sample inhomogeneities, like GaAs well width fluctuations.