In vertebrates, almost all somatic cells extend a single immotile cilium, referred to as a primary cilium. Increasing evidence suggests that primary cilia serve as cellular antennae in many types of tissues by sensing chemical or mechanical stimuli in the milieu surrounding the cells. In rodents an antibody to adenylyl cyclase 3 (AC3) has been widely used to label the primary cilia of neurons in vivo by immunostaining, whereas the lack of markers for the primary cilia of astrocytes has made it difficult to observe astrocytic primary cilia in vivo. Here, we obtained a visualization of astrocytic primary cilia in the mouse brain. In the somatosensory cortex, a large portion of neurons and astrocytes at postnatal day 10 (P10), and of neurons at P56 had AC3-positive primary cilia, whereas only approx. one-half of the astrocytes in the P56 mice carried primary cilia weakly positive for AC3. In contrast, the majority of astrocytes had ADP-ribosylation factor-like protein 13B (Arl13b)-positive primary cilia in the somatosensory cortex and other brain regions of P56 mice. The lengths of astrocytic primary cilia positive for Arl13b varied among the brain regions. Our data indicate that Arl13b is a noteworthy marker of astrocytic primary cilia in the brain.