Locomotion of the box jellyfish Chiropsalmus sp. (cf quadrigatus)1 (Haeckel) and Chironex fleckeri (Southcott) was analysed using digital video. Specimens of Chiropsalmus sp. and C. fleckeri were collected in 2001 and 2002, respectively, from coastal waters of Northern Queensland, Australia. Chiropsalmus sp. animals were videoed swimming in an aquarium, and C. fleckeri in a large outdoor tank. Locomotor sequences of nine Chiropsalmus sp. and seven C. fleckeri individuals were analysed using video techniques. A subset of animals had fluorescent dye injected into the sub-umbrellar cavity, to allow observation of water movements during ejection from the bell. Both species used an intermittent style of jet propulsion similar to that documented for some other species of cubozoan medusae. Computer analysis allowed examination of positions of bell parts over time intervals (0.04 s) by comparing coordinates of nodes marked on various bell parts using imaging software. Examination of node coordinates allowed a detailed qualitative description of gait, in addition to quantitative statistical analyses. General linear modelling showed that interspecific differences in locomotion were explicable in terms of body size. Larger animals of both species tended to swim faster, and with a lower pulse frequency, than smaller individuals. Smaller animals also tended to swim faster relative to their bell diameter.