We examined feeding preference in the threadsail filefish Stephanolepis cirrhifer on jellyfish and lobworm. Juvenile filefish were fed moon jellyfish Aurelia sp. and/or lobworm Perinereis nuntia vallata for 30 min in tanks with or without sand substrate, and the onset of foraging behavior, response latency (RL) and time spent foraging (total time spent searching for food and feeding) were analyzed using video recordings and measurements of food consumption. Organic matter consumption per unit of foraging time was also estimated for each prey item. In tanks with a sand substrate, RL was significantly shorter, foraging time was significantly longer, and organic matter consumption per unit foraging time was higher for jellyfish than for lobworm; however, the opposite trend was observed in tanks without a sand substrate. Filefish in all treatments consumed an average of 99±25% of their body weight in jellyfish in 30 min. This study revealed that although filefish prefer lobworm to jellyfish when both are readily available, filefish will select jellyfish when only jellyfish are visible. These findings imply that frequent jellyfish blooms increase the feeding opportunities of filefish on jellyfish.