ABSTRACT: The feeding dynamics and predation impact of the hyperiid amphipod Themisto gaudichaudi on mesozooplankton were studied in the vicinity of South Georgia, Southern Ocean, during austral summer 1994. Data show that T. gaudichaudi is a visualopportunistic predator, consuming primarily the most abundant species of copepods, euphausiids and pteropods. In situ estimated daily rations were equivalent to 6.3% of body dry weight, and similar to the value of 7.1% of body dry weight derivedusing an energy budget approach which takes into account the total daily energy requirements. In vitro estimates produced daily rations higher than these, ranging from 8.5 to 21.8% of body dry weight, increasing with prey density. The predationimpact of T. gaudichaudi adults, averaged over a 0 to 200 or 0 to 100 m layer, never exceeded 2.1% of mesozooplankton standing stock per day but accounted for up to 70% of the daily secondary production. This suggests that in the vicinity of SouthGeorgia T. gaudichaudi adults are able to control the local mesozooplankton community and may contribute significantly to the downward flux of biogenic carbon.