Scientific observation under the auspices of the CCAMLR Scheme of International Scientific Observatio was conducted onboard the Chilean longliner 'Puerto Ballena' fishing for Patagonian toothfish (Dissostichus eleginoides) in Subarea 48.3 from March to May 1996. Details of the cruise are provided. Biological investigations incorporated length, sex, maturity and weight measurements of D. eleginoides, the estimation of the finfish by-catch, the estimation of a conversion factor of product to fresh weight for D. eleginoides and the assessment of incidental mortality of seabirds associated with the longline operations. The catch of D. eleginoides was mainly comprised of specimens of 70-130 cm whith the proportion of females in the catch exceeding those of males in fish larger than 1 10 cm. Length at sexual maturity and length at first spawning differed considerably between males and females with males becoming mature at a much smaller length. Black-browed albatrosses and white-chinned petrels were the most abundant species in the vicinity of the vessel when longlines were set. About 44 % of the longlines were set during daylight hours. Consequently, black-browed albatrosses made up a considerable portion of the seabird by-catch. A by-catch of 159 dead seabirds was reported by the scientific observer and the crew with black-browed albatrosses and white-chinned petrels forming the bulk of the by-catch. In addition a number of albatrosses, giant petrels and white-chinned petrels were hooked alive during hauling and released alive. Sperm whales and Orcas were the most common marine mammals observed in the vicinity of the vessel. Orcas in some cases caused a considerable loss of fishes from the lines.