The present study describes the effects of various wavelengths of light on locomotion and byssal reattachment activities in the pearl oyster Pteria penguin, the most widespread cultured winged oyster for the production of valuable mabé pearls. Responses of pearl oysters to light have not yet been carefully observed and documented, despite the fact that light is known to influence the positioning of animals within the water column subject to local environmental conditions. To appropriately assess oyster movements and subsequent byssal reattachment, juveniles were filmed during unnatural light–dark cycles, alternating light and darkness every 6 h. Another experiment, to observe reactions to different light colors, was carried out by subjecting juveniles to either red or blue light during a continuous observation period of 24 h. Byssal reattachment for each juvenile was recorded at the end of each observation period. Light, independent of its color, strongly inhibited both locomotion and byssal reattachment activities in juveniles. Moreover, locomotion was observed to be unrelated to the shell size, but it is suggested that a large byssus may be responsible for inhibiting movement by preventing complete extension of the foot. High byssal reattachment rates to substrata under darkness in oyster juveniles is an important factor to take in consideration after detachment events during cleaning or when transplanting seeds to farm grounds.