Even the simplest bacteria and protozoa are capable of moving in response to stimuli. A Paramecium that finds itself in an unfavorable environment, such as water that is too warm or too salty, increases its speed and begins to make random turns. When it finds favorable conditions, such as cooler water, it slows and reduces its turning rate. These random, undirected movements are termed kineses (singular, kinesis).

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In contrast, taxes (singular, taxis) are movements in a specific direction in response to a stimulus. An interesting example of a taxis is movement oriented to a magnetic field, a behavior called magnetotaxis. The term was first used by the American microbiologist Richard Blakemore in 1975. Blakemore had found that anaerobic bacteria in the genus Aquaspirillum, which swim by means of flagella, tend to swim toward magnetic north. These bacteria can be attracted to the side of a dish with a bar magnet. Little bits of magnetized iron oxide, arranged in a row inside the bacterial cell, allow the bacteria to sense the magnetic field. Blakemore hypothesized that the bacteria swim north in order to swim deeper. In the Northern Hemisphere, the north magnetic pole is inclined downward. At Woods Hole, Massachusetts, where these bacteria were found, the magnetic pole is at about a 70-degree incline. Since the bacteria are anaerobic, they must stay buried in sediment, and they remain buried by moving downward along the magnetic gradient.

To test his hypothesis, Blakemore looked for and found bacteria in New Zealand that exhibited similar behavior to the Woods Hole bacteria except that they swam toward the south magnetic pole, bringing them downward in the Southern Hemisphere. Finally, he took New Zealand cultures back to Woods Hole, where the bacteria responded inappropriately to the unfamiliar magnetic field of the Northern Hemisphere, swimming up into the oxygen and dying. These experiments demonstrated that bacteria are able to sense a magnetic field and move in a directed fashion relative to that field.