Ultrafine new knob-like structures on the setulae of Daphnia were found by scanning electron microscopy. The size of the setular bosses or round knobs ranged from 20 to 66 nm in diameter, and the numbers varied from 23 to 138 per setula. The structures of the filter limbs 3 and 4 were studied in 19 populations of 12 Daphnia species, originating from lakes in South America (14), Europe(4), and Japan (1). A significant correlation was found between the length of setulae and the number of setular bosses.