The rotifer, Brachionus rotundformis along with a smaller strain were isolated for the first time from Cochin backwater. The newly isolated B, rotundformis has a length range of 98-210 microns showing a modal size of 168 microns, while in the smaller strain the length ranged from 70-168 microns with modal size at 140 microns. The lorica width to length ratio for B. rofundc;/orarmis is 0.7 and that for smaller strain 0.8. The smaller strains are of much use for feeding fish larvae especially those having small mouth size.