The sensitivity of thermal receptors and responses is compared with thermal noise in receptor cells and with thermal signals in the environment. It is demonstrated that the most sensitive responses known are far less sensitive than is physically possible but sufficiently sensitive to detect the smallest signals likely to be present in the environment. Expressions for the minimal thermal gradients detectable by organisms moving through them are derived. Thermal fluctuations in a receptor over physiologically relevant times and distances are almost certainly less than 10(-6) degrees C. The most sensitive responses reported in any organism are about a thousand times larger. The thermal gradient present in soil is nearly always greater than 10(-3) degrees C/cm and it is probably higher in other environments. The suggestion that nematodes locate plant roots based on heat production is shown to be unlikely because the gradients produced are smaller than those from other causes. Bacteria, a slime mold, rattlesnakes, and mammals are discussed in addition to nematodes.