More white-chinned petrels (Procellaria aequinoctialis) are accidentally killed in fisheries than probably any other seabird in the world, but the population impact of this mortality is poorly understood, partly because there have been no estimates of the species’ abundance in recent decades. The largest breeding aggregation, comprising the majority of the worldwide total, is believed to be on the sub-Antarctic island of South Georgia. We estimated the size of this population by calculating the area of suitable habitat and the density of occupied burrows within it. Just less than one million pairs of white-chinned petrels laid on South Georgia in the survey seasons (2005/06 and 06/07). This is 50% of the previous estimate, but still represents around two-thirds of the global population. If the population is declining due to fishery bycatch off S America, as is likely, the scale of annual mortality in this population alone is at least in the high tens of thousands, and plausibly hundreds of thousands.