Predation on adult microhylid frogs Kaloula pulchra by two closely-related colubrid snakes is described, based on two observations of Oligodon formosanus in Hong Kong and one observation of O. fasciolatus in Thailand. In two instances, O. formosanus was observed cutting open the abdomen of this anuran species. In one case, it performed repeated rotations about its own longitudinal body axis (“death roll”) while its head was inserted into the frog’s abdomen. The purpose of this behaviour was probably to tear off organs and swallow them. Once O. fasciolatus was observed catching and swallowing K. pulchra whole. In that case, the snake also made a series of rotations while it maintained its firm grip in the frog’s belly. It is concluded that, for these two closely-related kukri snakes, prey size is crucial for determining whether the gape width allows large preys to be swallowed entire.