ABSTRACT Photoreactive behaviour of Elysia timida was examined with regard to the posture of its parapodia, covering or exposing its symbiotic chloroplasts. In the dark, the parapodia are held dorsally close together, but they open and spread out in light, at 3 × 103 to 3 × 105 ergs. cm−2. s−1. At higher light intensities (up to 6×105 ergs. cm−2. s−1) the parapodia close up again. This behaviour is identical in normal and eyeless individuals. At a wavelength of 540 nm, the slugs open their parapodia as above, but the eyeless E. timida will not close their parapodia at the higher light intensities at this wavelength. At 445 and 650 nm, no photoreaction is observed. It is postulated that E. timida has three photoreceptor systems. (1) An extraocular photoreceptor that differentiates between light and dark, which causes, respectively, opening or closing of parapodia. (2) The eyes, which can differentiate between optimal and above optimal light intensities (the latter causing closure of the parapodia). These photoreceptors perceive light at 540 nm, and could thus contain carotenoid photoreactive pigments. (3) An extraocular photoreceptor, which, like the eyes, can differentiate above optimal light intensities and cause parapodial closure, but only in ‘white’ light. An interaction of the first two photoreceptors could be an alternative for the function of the latter. The form or location of the extraocular photoreceptors is unknown. It is proposed that although the symbiosis of algal chloroplasts and E. timida might be of recent evolution, this symbiosis has already affected the behaviour of the slug.