When a layer of heavy matter is above lighter matter in a star, the inverse $\mu $-gradient may lead to thermohaline (or double-diffusive) convection. This has been studied in the past for helium-rich atmospheres, but it may also occur for metal-rich layers. It has recently been studied for the accretion of hydrogen poor material onto the host stars of exoplanets. These stars present a metallicity excess compared to stars in which no planets have been detected. However, the reason for this excess is still a subject of debate. It may be primordial or the result of accretion, or both. In this last case, thermohaline convection may lead to “metallic fingers” which partially dilute the accreted matter inside the star. Such an effect can also be important in the chemically peculiar A stars in which metals accumulate in the atmospheric layers.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html