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The control of rats in the urban environment involves more than merely dealing with the above-ground populations. The average urban sewage system provides a vast labyrinth of passages and nesting places for the Norway rat, Rattus norvegicus. Here, in a protected underground habitat the rodent population is free to reproduce and ultimately expand to the maximum number that the environment can support. Eventually, the population outgrows its environment. At this point population pressures force animals to move out of the system. Rat burrows begin appearing in front yards, under sidewalks and driveways, and in flower beds, and rats themselves are occasionally seen emerging from drains and swimming into toilets. The result may be a new colony of rats on the surface and almost certainly numerous complaints to the control agency. In short, the uncontrolled populations of rats in sewers constitute a reservoir which extends into all parts of a city and is capable of establishing new infestations on the surface.