With a Brewer spectrophotometer, an estimation of total ozone is made from relative measurements of direct-sun ultraviolet radiation at six wavelengths from 300 to 320 nm. During normal operations, one of six neutral-density filters is selected automatically to maintain the detector in its linear response range. On the basis of these standard direct-sun observations, estimates of aerosol optical depth can be derived, provided that a calibration of the relative measurements is available for each neutral-density filter. To obtain the calibration, we implemented a routine to measure direct-sun signals with a fixed neutral-density filter and applied the Langley method to the measured photon counts. Results show that if a sufficiently large number of cloud-free mornings or afternoons is available, a reliable calibration can be achieved even at sea-level sites that are characterized by large aerosol variability. The derived aerosol optical depths appear consistent with those measured independently by a multifilter rotating shadow-band radiometer. Existing relatively long-term series of direct-sun ozone measurements by Brewer instruments may be used for retrieval of aerosol optical depth.