ABSTRACT.Electromyograms from the abdominal dorso‐ventral muscles of a freely swimming water‐bug,Lethocerus cordofanusMayr (Belostomatidae), have shown that during a normal dive ofc.30 min duration there is a gradual rise in the frequency of tonic firing of slow motor axons which brings about a compression of the abdomen. This may help to offset a small, measured, negative pressure due mainly to nitrogen loss from the ventral hairpile which is thought to act as a physical gill. Towards the end of a dive, fast axons become tonically active and surfacing behaviour occurs. These responses can be experimentally manipulated by adding or extracting gases from the subalar store of a submerged bug. Similar responses have been obtained by subjecting a bug to high external pressures in a sealed vessel, and they suggest that a bug does not normally descend deeper than 5 m. It is also shown that either hypoxia or a reduction of store volume can initiate surfacing behaviour, although the latter is probably the normal stimulus for ascent. Muscular compression of the abdomen may help to prevent the inward leak of water into the store during a dive. It may also help to ensure the continuity of the lateral pathway between the ventral hairpile and the subalar store by slightly depressing the abdominal margins. The receptors controlling the motor response do not measure movements of an air—water interface, and they are probably internal proprioceptors, although their location is un