In order to expand the practice of rearing marine finfish larvae, it is necessary to increase and stabilize the production of live feed. Latest researches on rotifer culture technology in Japan have aimed at these objectives. This paper summarizes the progress on the technology of high-density rotifer mass cultures. The high-density culture of the marine rotifer Brachionus rotundiformis (so-called S-type) enables production that is at least 100 times more efficient than that by the conventional method, with less cost for production. The technology is based on the use of highly concentrated algal biomass as food for rotifers. Rotifers are mass-cultured at 10000-30000 individuals mL-1 by feeding them exclusively on condensed freshwater Chlorella. Cultures are generally supplied with constant O2; pH is adjusted to 7 by addition of HCI to avoid an increase of free ammonia in the culture system. The excess suspended organic matter can be efficiently removed by a nylon filtration mat.