Several sacoglossan sea slugs utilize chloroplasts taken from food algae for photosynthesis (termed kleptoplasty). So far, no studies have revealed the relative importance of light and food or the interaction between these factors. In this study, two sacoglossans (Elysia trisinuata and E. atroviridis ) were individually reared under four experimental conditions (combination of light/dark and with/without food) for 16 days, and their survival rate and relative (= final/initial) weights were measured. Both light and food had positive effects on survival in E. trisinuata , whereas no positive effects of light or food were detected in E. atroviridis . Both light and food had positive effects on relative weights in both species, and light had weaker effects than food. A significant interaction term between light and food was detected in E. trisinuata (but not in E. atroviridis ); only the presence of light and food resulted in increased weights. This result suggests that E. trisinuata must obtain fresh chloroplasts from algae to keep photosynthetic activity. This possibility was confirmed by the observation that unfed E. trisinuata had a shorter duration (< 4 days) of photosynthesis than unfed E. atroviridis .