Biological control is a cheap, permanent, and nontoxic method of controlling aquatic weeds. Insects and fish are the most effective biological agents. The use of herbivorous fish in the United States to control the growth of aquatic vegetation has been limited because of the susceptibility of most of these fish to low temperature. However, the amur is an herbivorous fish that can tolerate temperature extremes. The amur has a voracious appetite and consumes many aquatic plants. It has been successfully introduced in other countries of the world, and, in the future, may be used to mitigate some of the aquatic weed problems prevalent in the United States. Many fishes, snails, manatees, and other organisms can be expected to be effective only in tropical or subtropical waters. Their usefulness in temperature regions will be sharply limited by minimum temperatures. The use of viruses, fungi, bacteria, and other pathogens offers a potential for biological control of aquatic weeds. Low growing aquatic plants may prevent the establishment of some problem plants.