Keywords: Daily growth rate in situ fluorescent marking Intertidal mudflat Lucinidae Mark-and-recapture Seagrass The fluorochrome calcein has been used in numerous growth studies of molluscs to internally mark calcified structures. Because of interspecific variations in marking success and possible effects on growth performance, methodological assessments of the suitability of calcein as a growth marker, especially in field contexts, remain necessary. Here we report on the effects of different calcein concentrations (100, 200, 400 and 800 mg l −1) on fluorescent mark deposition, growth rate, density, body condition and size–frequency distribution of an intertidal infaunal bivalve species, Loripes lacteus (Linnaeus, 1758), using an outdoor immersion technique. To avoid stress caused by handling and transportation, in situ enclosures were placed at seagrass-covered patches during low tide, to which calcein solutions were added. After a 1.3-to 2.6-h period of exposure to calcein, the enclosures were removed. Sites were sampled three months later. All calcein concentrations produced live L. lacteus with a clear fluorescent shell mark, but the percentage successfully marked tended to increase with higher calcein concentrations. Furthermore, marking success and growth rate decreased significantly with shell size (i.e., age). Calcein concentration did not measurably affect shell growth rate, body condition and size– frequency distribution, but the numerical densities were lower for sites treated with calcein concentrations ≥ 400 mg l −1 . Our results suggest that in situ calcein-marking of burrowing lucinid bivalves with low concentrations (100–200 mg l −1) is a non-invasive and rapid method to determine growth rate, provided that the lucinids are not too old.