We present the first direct measurements of the electric field and plasma density with high resolution in Short Large-Amplitude Magnetic Structures (SLAMS) at a quasi-parallel shock crossing. Cluster observations at the Earth's bow shock confirm the fast-mode structure of SLAMS. Using the four Cluster spacecraft, we determine the velocity of the SLAMS vSLAMS and find that E = -vSLAMS ¿ B. This indicates that locally the SLAMS is moving with the same velocity as the plasma, thus indicating trapping of ions. A small-scale (electron gyroradius) electric field with E ≠ -vSLAMS ¿ B is found at the leading edge of the SLAMS. Downstream of several SLAMS, a wake-like decrease in the plasma density is observed suggesting the three-dimensional structure of SLAMS. The scale length of these SLAMS is of the order, or larger, than the satellite separation of 600 km.