By using low-temperature, clean, and nutrient-rich properties of deep seawater (DSW; seawater below the euphotic layer), a continuous and simultaneous cultivation system for a benthic food diatom, Nitzschia sp., and juvenile abalone was established. Cell suspension of Nitzschia sp. was added to a bioreactor made of acrylic pipe (7 cm diameter x 50 cm long) containing short vinyl tubes (2 cm diameter x 2 cm long) as substrata. DSW collected from 320 m depth at Muroto City, in Kochi Prefecture, Japan, was supplied to the reactor and incubated under natural light (ca. 6000 lux) with a continuous DSW flow rate of 40 turnovers per hour. After growing enough benthic diatoms in the reactor, juveniles of abalone, Haliotis sieboldii (shell length ca. 10-20 mm) were put into a reactor, and cultivated simultaneously with food diatoms in the continuous flow system. During the four-month incubation, 7-month-old abalone juveniles with a shell length of 12.4 (average) +/-0.2 (SD) mm were grown to 19.4 (+/-1.7) mm in the reactor. Daily growth rates of abalones were 50-110 µm/day. These results indicate that the continuous cultivation system with DSW supports the growth of juvenile abalone without any supply of seaweed until it grows to release size.