The effect of lighting to rotifer culture water was investigated preliminarily using freshwater Chlorella as food in terms of vitamin B₁₂ (VB₁₂) production. The marine rotifer Brachionus plicatilis was batchcultured for five days at 25°C in 25 psu seawater with or without lighting (L:D = 13:11 and L:D = 0:24). Rotifers were fed VB₁₂-enriched or VB₁₂-free Chlorella vulgaris with or without cobalt compound supplementation (cobalt (II) sulfate heptahydrate: CoSO₄•7H₂O). When VB₁₂-enriched Chlorella was fed, rotifer with lighting showed better population growth than in complete dark. On the other hand, no difference was observed in the population growth between light and dark groups when VB₁₂-free Chlorella was used. Nevertheless if the cobalt compound was supplemented to VB₁₂-free Chlorella, the light group showed higher population growth than non-supplemented groups and dark groups, and a much higher amount of VB₁₂ was detected from the tanks than those from other groups. In addition to that, even in the group fed VB₁₂-free Chlorella without Co supplementation a daily increase of a small amount of VB₁₂ was observed when they were lit up. From these experimental results, we can conclude that lighting plays an important role for the population growth of rotifers and Co compound supplementation to rotifer culture water promotes the reproduction performance of rotifers due to the enhanced production of VB₁₂ by lighting.