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.