The flag pen shell Atrina vexillum is probably one of the most economically important pinnid species in the Indo-West Pacific. Its populations have suffered a threatening decline during past decades. Despite the need for conservation, knowledge of the ecology of this pinnid is practically absent. The condition of the mollusc population is generally inferred from age- and growth-related data of the investigated mollusc. A combination of Mg:Ca ratios, growth increments on the external shell surface, and annual muscle scar rings on the inner shell surface were used to estimate the age and growth rates of 4. vexillum from 3 subareas in the Gulf of Thailand along the southwestern coast of Vietnam. This study showed that the molluscs were ≤10.5 y old and had a shell length of ≤438 mm. Shell growth rates had a seasonal pattern, with an extended period of very slow growth during the cold season and a short period of slow growth during late spring and summer. The growth rates of A. vexillum varied greatly with subareas. In moving inward in the Gulf (toward its top), the A. vexillum growth rates were slower and its life span was shorter. There were statistically significant differences between subareas in environmental parameters, such as water salinity, temperature, concentrations of suspended particulate matter and suspended organic carbon, chlorophyll a, and oxygen concentration. In the inner subareas, A. vexillum is subject to quite a set of unfavorable environmental factors, such as lower and more changeable water salinity, warmer water temperature, greater concentration of suspended particulate matter, and lower oxygen concentration in the water compared with the outer subarea of the Gulf. The obtained data are a first contribution to the knowledge of the ecology of this little-studied mollusc.