A collaborative project was established between the Fisheries Research Institute, National Taiwan University and CSIRO in Australia studied the growth dynamics of longtail tuna in two spatially separated neritic regions during 2005–20010. In total, 686 fish were collected: 245 from Taiwan (30–75.3 cm FL; 0.45– 9.9 kg) and 461 fish from Australia (23.8–125.0 cm FL; 0.26–27.8 kg). In Taiwanese waters, fish ranged in age from 86–273 days (0.2–0.7 years), with the entire catch being <1 year of age. In contrast, fish ages in Australian waters ranged between 0.4–18.7 years with the majority of fish being 4–9 years of age and only one juvenile fish was captured. This study has revealed that the two regions support very different age structures of fish, which may be due to several factors that need further investigation within a collaborative research program. Some of these factors may be: i) longtail tuna comprise a single stock throughout the Western Pacific Ocean where the waters around Taiwan serve as a nursery before fish move south to Australia as they grow, ii) longtail tuna are only temporary residents in Taiwanese waters as they migrate to waters of southern Japan, iii) a mis-match of spatial and/or temporal sampling effort for longtail tuna or selectivity of sampling gears around Taiwan may have contributed to an underrepresentation of the largest and smallest fish present in the population. Continued sampling of longtail tuna in Taiwanese waters for age and growth determination and a collaborative study between Taiwan and Australia on longtail tuna stock structure will begin to fill some of the large knowledge gaps that currently exist for this little-studied tuna species.