THE Asiatic grass carp is a herbivorous fish which is being bred and extensively cultured for food in Russia and eastern Europe. In Britain where freshwater fish are seldom eaten, apart from eels and salmonids, grass carp are arousing interest because of the possibility of using them as a supplement, or in some situations as an alternative, to present methods of weed control—mechanical cutting or the use of herbicides1. The chief factors involved in the possible use of grass carp have been discussed2,3, one of the most important being the density of fish needed to achieve a desired degree of control. We report here the work done on this topic in 1969.