The oviposition of female locusts is a complex behaviour that includes a dramatic extension of the abdomen. The role of internal pressure during oviposition was investigated by monitoring the intra-tracheal pressure and the activity of selected longitudinal muscles, while movements of the abdomen were visualised with a video imaging system. Locust oviposition consists of a sequence of four distinct phases: (i) probing the substrate and digging without elongation of the abdomen, (ii) longitudinal extension of the abdomen up to four times its normal length, (iii) laying packages of eggs while (iv) gradually withdrawing the abdomen. During extension, neurograms and myograms of selected longitudinal muscles revealed a decreased level of activity. When the abdomen retracted to its normal length, muscle activity re-appeared. In phases two and three, rising internal pressure prevented the abdomen from slipping back when the valves released their lateral grip from the substrate. Locking the genital segments in the hole by relative bending kept the abdomen in place when producing foam or laying eggs. Intra-abdominal pressure, therefore, is not the main cause of abdominal extension, but rather maintains extension when no mechanical locking in the hole prevents the abdomen from elastic retraction.