MASSIVE stars that explode at the end of their lives (type II supernovae) have been generally thought to do so rather isotropically, but there have been very few near enough to our Galaxy to study in reasonable detail. Supernova 1993J, in the nearby galaxy M81, is the second-closest supernova since the invention of the telescope, which enables us to study its early evolution. Here we report the detection of blue-shifted oxygen lines only four months after the optical discovery of the supernova. Unlike the case of SN1987A (refs 1, 2), we suggest that this blue shift is not the result of dust formation, but probably arises because the photosphere of the supernova developed a complex, irregular structure near maximum light. The clumpy ejecta allows photons from oxygen on the near (blue-shifted) side of the photosphere, which would not be visible if the ejecta were smooth and isotropic, to escape and be detected, while the photons on the far (red-shifted) side of the supernova are still absorbed. This situation arises because of mixing of regions of different element groups(3,4), and clearly shows the importance of instabilities during the supernova explosion.