Developing cartridges include a main body, a developer transport member, and a developer thickness regulating member. The main body includes a first and second side portions separated from each other along a direction. The developer thickness regulating member regulates a developer thickness and extends in a direction between the first and second side portions. The developer thickness regulating member includes an end portion, which is disposed near the first side portion, and a first protrusion, which extends along the particular direction from the end portion to at least the first side portion. The main body forms a cavity in the first side portion. The cavity is defined by at least a first surface, a second surface opposing the first surface, and a second protrusion that protrudes from the first surface toward the second surface. The first protrusion is fixed at a position between the second protrusion and the second surface.