The swimming behaviour and photoresponses of the iridescent epipelagic copepods, Sapphirina gastrica and S. opalina were investigated in the laboratory. In continuous dark conditions, both species showed no significant diel variation in their swimming activities. When stimulated with light, they exhibited spiral-swimming in which the males showed a significantly higher speed and frequency of turning than the females. Both sexes of S. gastrica and S. opalina showed positive phototaxis at intensities higher than 0·05×1014 quanta  cm−2 s−1 for light sources of 430 nm and 580 nm. Sapphirina gastrica showed increased locomotion over a broad range of wavelengths from 430 nm to 580 nm, while S. opalina showed a gradual increase of activity with decreasing wavelength, with the highest value at 430 nm. The photoresponses of these two species suggest that light conditions play an important role in their daytime ascent and in determining the depth distributions that were observed in our previous study. It is suggested that the iridescence and fast spiral-swimming of males, and the species-specific photoresponses of both sexes constitute a putative mate recognition system in the open ocean.