ABSTRACT: Planktonic larvae of the eastern oyster Crassostrea virginica are able to regulate their vertical position in the water, but the environmental cues responsible for this regulation, particularly in turbulent settings, remain unclear. We quantified swimming responses of late-stage oyster larvae in a grid-stirred turbulence tank to determine how light affects the swimming behavior of larvae over a range of hydrodynamic conditions similar to their natural coastal environments. We used particle image velocimetry and larval tracking to isolate larval swimming from local flow and to quantify 3 behavioral metrics: vertical swimming direction, proportion of larvae diving, and proportion of larvae swimming helically. We compared these metrics across turbulence levels ranging from still water (ε = 0 cm² s^-3) to estuarine-like conditions (ε = 0.4 cm² s^-3) in light and dark. At all turbulence levels, light had no effect on the proportion of upward swimming larvae, but elicited detectable increases in the proportion of helical swimming and diving behaviors. We further examined the effect of light and turbulence on specific characteristics of helical trajectories, and found that these environmental cues induce changes to both vertical and horizontal velocities of helically swimming larvae, changing the helix geometry. The increased prevalence of these behaviors in light likely plays an ecological role: increased diving in light (in conjunction with turbulence) is a potential mechanism to enhance settlement success, while changes to helical swimming in light may serve an anti-predatory function. Together, these behaviors provide insight into potentially complex larval responses to multiple simultaneous environmental cues.