The objective of the present study was to perform a more detailed investigation of the brain morphology of adult C3aR−/− mice, determine whether the previously reported disordered layering of the developing cortex due to disturbed C3aR signaling persists into the adulthood, and examine the behavior of C3aR−/− mice with focus on locomotor activity, motor function, memory, and autism-like repetitive behavior. Here, C3AR1 is linked to autism.