Algae used to be considered plants, but the world is no longer divided into “animal, vegetable, or mineral.” Now, phytoplankton (algae) and zooplankton (protozoa) are both placed in a new fifth kingdom called Protoctista, giving us the present classification into bacteria, fungi, protoctists, plants, and animals. Algae are often one-celled and microscopic in size. They produce three types of problem, two of them serious. Several kinds of microscopic one-celled and filamentous algae produce the “musty-earthy-mildewy-moldy-fishy” taste and odor which ranks No.2 in consumers’ complaints of tap water (after chlorine T&O). Some of the same algal species are also capable of causing serious clogging of filters. During seasonal algae blooms in reservoirs, their numbers may be so enormous that the municipal filtration plant output is less than half of actual production-they use more water backwashing the filters than they put into the distribution system. When they take short-cuts at the waterworks, point-of-use filters also clog quickly. The less serious problem is toxicity. Two or three common species produce both nerve and liver toxins that can be deadly. This isnot a serious problem because the water does not become dangerous until after it has become so smelly and unsightly as to be disgusting to a person. Thus, pets and farm animals may be affected by algal toxins in ponds and puddles, but hardly ever people. Algal toxins are not presently regulated, but they are being watched carefully.