Mackerel icefish (Champsocephalus gunnari) are widespread on the South Georgia shelf, Antarctica, and have been fished commercially since the early 1970s. They are known to feed predominantly on krill. An index of condition which uses the ratio of the measured total mass to the estimated mass is shown to provide a good indicator of local krill density. The index is likely to be little affected by the reproductive cycle unless there is high krill availability during the months around the spawning time, and even then the effect is much less than the highest observed values. The condition index responds rapidly to changes in krill density and therefore can provide indications of short-term variations in krill availability. Condition index provides a useful proxy for krill density and is likely to be of considerable value in interpreting the results from ecosystein assessments such as that in progress under the auspices of CCAMLR.