Marine zooplankton, rotifers are the most important live food and play an indispensable role as an initial food item in larviculture of marine animals. At present, only two species of rotifer, Brachionus rotundiformis (S-type rotifer) and Brachionus plicatilis (L-type rotifer) are used as the larval food. The bogy size of the L-type rotifer is about 3 times larger than the S-type. Improvement of the culture technology for mass production of rotifer has been important for development of aquaculture. However, population growth density of rotifers in mass culture was only on the level of hundreds individuals (ind)/ml in hatcheries. For the S-type rotifer, we developed high-density culture method with diminishing the inhibitory factors to rotifers. The method enabled to cultivate the S-type rotifer at the density over 2x10⁴ ind/ml. We now focus on the production of the L-type rotifer. In this study, high-density culture method for the L-type rotifer was investigated in preliminary trial using a 2 l-scale culture vessel on basis of the culture method of the S-type. The culture method for the L-type rotifer was improved by using the following techniques: pH-stat to promote the dissociation of toxic NH₃ to the less toxic NH4⁺+ OH^-, supply of oxygen gas to avoid shortage of dissolved oxygen, continuous feeding of Chlorella paste at short intervals, and removal of particulate organic matters with a nylon mat. As the result, very high culture density over 3x10⁴ ind/ml was obtained in case of the L-type rotifer.