Record low ozone was measured by balloon-borne ozonesondes (40 flights) at McMurdo Station, Antarctica (78 ¿S) during the 1993 austral spring. Total column ozone declined by 55% from an initial 275 Dobson Units (DU) on 30 August to a minimum of 130¿7 DU on 2 October. Ozone within the 12--20 km showed a 95% decrease from an initial 138 DU in August to a record low 7 DU on 19 October. Probable causes of the 1993 record low ozone, based on balloon-borne observations at McMurdo include: the presence of the Pinatubo volcanic aerosol layer between 11 and 16 km (though decreased from the 1992 season); a colder than normal stratosphere over McMurdo (183 K minimum); and a relatively stable polar vortex which delayed the intrusion of high levels of ozone from outside the polar vortex wall until after 22 October. These conditions provided an optimum environment for the formation of polar stratospheric clouds (PSCs), essential to the heterogeneous chemistry that subsequently leads to the catalytic destruction of ozone by reactive chlorine. ¿ American Geophysical Union 1995