Data collected by Australian observers, combined with logbook data supplied by all Japanese vessels fishing in the Australian Fishing Zone were used to estimate the total seabird catch and catch rates (and associated variances) within the Australian Fishing Zone by season and area. The species composition of subsamples of captured seabirds was used in conjunction with the estimates of total by-catch to provide estimates of by-catch by species. The total by-catch of all seabird species by Japanese longline within Australian waters was estimated to be 2,981 (cv 17%) for the 1992 fishing year, 3,590 (cv 15%) in 1993 and 2,817 (cv 19%) in 1994. However, the observed seabird bycatch may underestimate the total number of seabirds killed by 27%. The increase in total seabird by-catch between 1992 and 1993 is partly due to the use of monofilament nylon longline gear by two vessels in 1993, and the decrease from 1993 to 1994 was mainly due to a corresponding reduction in fishing effort. For the three years examined, 78% of the total seabird by-catch was albatrosses, with black-browed albatross (Diomedea melanophrys) and shy albatross (D. cauta) caught in the greatest numbers. Catches of yellow-nosed albatross (D. chlororhynchos), wandering albatross (D. exulans}, and grey-headed albatross (D. chrysostoma) were also substantial. The proportions of individual species caught showed considerable inter-annual variability. Most seabirds caught and killed by longline fishing are captured during line setting. Data collected by the observers were also used to determine the influence of various environmental factors and mitigation measures on seabird catch rates. Generalised linear models were used to determine the statistical significance of the effect of each factor on the seabird catch rate. Results show that the environmental factor that has the most influence is whether line setting was carried out at night or during the day. For the data examined, the chance of catching seabirds during day sets was five times greater than for night sets. For night sets, the chance of catching seabirds during the full half-phase of the moon was five times greater than during the new half-phase. Other environmental factors with significant effects were the area and season fished. Wind, cloud and sea conditions were not found to have a significant influence. Considerable variation in the seabird by-catch rate among vessels was found, which is probably due to differences in their implementation of mitigation measures, as well as the clumped distribution of seabirds by area and time. Interannual variation in the by-catch rate was found to be statistically significant but the differences among years was small in comparison to other factors. An examination of the influence of mitigation measures for sets made during the day in summer in the Tasmanian area showed that the level of bait thawing and unidentified factors related to individual vessels were most significant in determining the seabird by-catch rate, followed by the use of a bait throwing device. It was not possible to examine the influence of the use of bird scaring tori poles and lines as these were used during all sets examined in detail. For this data set, the amount of cloud cover had an influence, while moon phase, sea conditions and wind strength did not.