Members of Tigriopus californicus, and harpacticoid copepod, live in small, shallow tidepools in the upper spray zone where they cannot avoid the full effect of visible and ultraviolet (UV) radiation. Field experiments using ambient light show that individuals of T. californicus aggregate in areas of lower radiation at midday, yet have no preference between areas exposed only to photosynthetically active radiation (PAR) or shade, but aggregate in the shaded portion of a tank when exposed to UV_B. Light detection in T. californicus is presumably by the nauplius eye, which is described at the histoligical and iltrastructural levels. Microvilli of the photosensitive rhabdomere are regularly packed at noon, dusk, and midnight. Processes suggesting rhabdomere synthesis, including vesicles and tubules binding to the base of microvilli, are observed at these times. At dawn, the rhabdomere shows areas of degeneration and coated pits and multivesicular bodies are common at the base of the microvilli. Comparison with previous studies show a wide variety of nauplius eye complexity in copepods.