We determine the extinction curve in the z_l=0.83 lens galaxy of the gravitational lens SBS0909+532 from the wavelength dependence of the flux ratio between the lensed quasar images (z_s=1.38) from 3400 to 9200AA. It is the first measurement of an extinction curve at a cosmological distance of comparable quality to those obtained within the Galaxy. The extinction curve has a strong 2175AA feature, a noteworthy fact because it has been weak or non-existent in most estimates of extinction curves outside the Galaxy. The extinction curve is fitted well by a standard $R_V=2.1pm0.9$ Galactic extinction curve. If we assume standard Galactic extinction laws, the estimated dust redshift of $z=0.88pm0.02$ is in good agreement with the spectroscopic redshift of the lens galaxy. The widespread assumption that SMC extinction curves are more appropriate models for cosmological dust may be incorrect.
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