The medial entorhinal cortex (EC) is a part of the neural network for the representation of self-location in the rat. The key cell type of this system is the grid cell, whose multiple firing fields span the environment in a remarkably regular triangular or hexagonal pattern. The basic properties of grid cells and other cell types have been described, but the neuronal mechanisms responsible for the formation and maintenance of the place code remain elusive. These mechanisms can be investigated by genetic intervention strategies, where specific components of the entorhinal-hippocampal network are activated or silenced. Because of the common use of knockout mice for such targeted interventions, we asked if grid activity is expressed also in the mouse. Principal neurons in the superficial layers of mouse medial EC had stable grid fields similar to those of the rat. Neighboring grid cells shared a common spacing and orientation but had a different spatial phase, such that a small number of grid cells collectively represented all locations in the environment. The spacing of the grid increased with distance from the dorsal border of the medial EC. The lowest values for grid spacing, recorded at the dorsal end, were comparable to those of the rat, suggesting that grid fields do not scale up proportionally with body size. Grid cells were colocalized with head-direction cells and conjunctive place x head-direction cells, as in the rat. The demonstration of grid cells in mice prepares the ground for transgenic analyses of the entorhinal-hippocampal network.