Monitoring of environmental and tissue pH is crucial for the survival of animals. The nematode Caenorhabditis elegans (C. elegans) is an excellent model organism for the analysis of neural circuits that regulate animal behaviors. The animal is attracted to mildly alkaline pH up to pH 10, and avoids strongly alkaline pH higher than pH 10. Our genetic dissection and calcium imaging demonstrated that ASEL and ASH are the major sensory neurons responsible for attraction to mildly alkaline pH and repulsion from strongly alkaline pH, respectively. In ASEL, a transmembrane-type guanylyl cyclase, GCY-14, is activated by environmental alkalinization, and in turn, a cGMP-gated cation channel serves for Ca2+ influx into the sensory neuron, which induces ASEL activation. In ASH, TRPV channels are found to be required for the neural activation upon stimulation with strongly alkaline pH. To understand the animal's behavioral switch at molecular and cellular levels, we have analyzed behaviors of mutants defective in ASEL and/or ASH under various alkaline pH, and have found that activities of ASEL and ASH compete each other for the behavioral switch. While mildly alkaline pH preferentially activates ASEL, strongly alkaline pH activates both ASEL and ASH, and ASH activity overrides the activity of ASEL. Neural circuits responsible for this decision making will also be discussed.