Tagging methods used in bivalve research can be broadly categorized into two groups: internal and external. External tagging methods are more commonly used; however, the legibility of external tags tends to decline over time, and tag recovery rates are often low due to the abrasion or biofouling of shells, particularly during long-term studies. The aim of the current study was to compare two external identification methods placed on or in the shells of adult Crassostrea gigas: (1) a plasticlaminated glue-on tag and (2) novel use of a t-bar anchor tag inserted into the upper shell, to determine the optimal methodology to use in longitudinal studies of greater than 1-y duration. Over a 15-mo period, 100% of glue-on numbered tags were lost or became overgrown and could not be read, whereas 91.5% of t-bar anchor tags were retained and remained legible. The results of this study suggest that t-bar anchor tags are a better choice for long-term longitudinal studies of adult C. gigas in temperate marine conditions than plastic-laminated labels glued to the exterior of the shell. These findings may be applicable to other bivalves.