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This paper describes the design of a fast long-jumping robot conceived to move in unstructured environments through simple feed-forward control laws. Despite the apparent similarities with hopping, jumping dynamics is peculiar and involve non-trivial issues on actuation powering, energy saving and stability. The "Grillo" robot described here is a quadruped, 50-mm robot that weights about 15 grams and is suited for a long-jumping gait. Inspired by frog locomotion, a tiny motor load the springs connected to the hind limbs. At take-off, an escapement mechanism releases the loaded springs. This provides a peak power output that can exceed several times the maximum motor power. In this way, the actuation and energy systems can be significantly reduced in weight and size. On the other hand, passive dynamics is exploited by compliant forelegs, that let to partially recover the impact energy in their elastic recoil. Equipped with a 0.2W DC motor, the robot is dimensioned to achieve a forward speed of 1.5 m/s, which corresponds to about 30 body length per second.