The medial entorhinal cortex (MEC) contains parvalbumin (PV)- and somatostatin (SOM)-expressing interneurons, but their individual roles in grid formation by grid cells in the MEC is poorly understood. The authors recorded MEC grid cell activity in freely moving mice while selectively silencing PV-expressing interneurons and found that it interfered with the spatial selectivity of grid cells and the tuning of speed cells. By contrast, silencing of SOM-expressing interneurons had no effect on speed cells or grid cells but decreased the spatial selectivity of cells with aperiodic firing fields, indicating that these two interneuron subtypes have different roles in spatial representation in the MEC.