This paper describes the interaction between sperm whales (Physeter macrocephalus) and the toothfish fishery (Dissostichus eleginoides) carried out by longline fishing vessels operating in two different fishing zones at 40º and 50º latitudes in the Southwestern Atlantic from March to May 2007. 62 hauls were performed in the northern zone (latitude 40º 00’ S), while 41 hauls were performed in the southern zone (latitude 50º 00’ S) at an average depth of -1282 m. 57.2% of the total number of hauls were observed. Onboard observers recorded: a) quantity and quality of fish parts remaining in the recovered longline; b) presence and number of sperm whales and c) comparative fishing yields with and without sperm whales effective predation. The observations were performed using both, the traditional Spanish longline and the Mammals and Birds Excluding Device (MBED). The longline with MBED sinking rate was determined in 1.14 m/seconds. In both fishing zones, the sperm whales presence was observed in 77.4 % of total observed sets and the effective predation was determined in 22.6 % with MBED. Effective predation was determined in 44 % of observed sets during the day period from 12:00 to 18:00 hours GMT. Lips and buccal parts were observed in 71 % of sets with traditional longline and in 27 % with MBED. The estimated fishing yields of northern and southern zones were 11.05 kg/hour and 15.53 kg/hour respectively, using the MBED and with sperm whales effective predation. In the southern zone the fishing yield increased to 23.03 kg/hour, using MBED but without sperm whale evidence of effective predation. No incidental mortality of birds was registered using both tory-line and MBED simultaneously, in spite of remarkable abundance of the southern black-browed albatross (Thalassarche melanophrys) and cape petrel (Daption capense) in 40.76 % and 23.13 % of total observed sets respectively.