Viguiera dentata (Cav.) Sprenguel (Asteraceae) is known as “chimalacate” in several Mexican states. Infusions of the aerial part of this species are used for the treatment of baby rash by the inhabitants of San Rafael, Coxcatlan, Puebla, Mexico. We examined the antibacterial and antifungal activities through the disk-diffusion method. The composition and antimicrobial activity of the essential oil of V. dentata is reported here for the first time. The essential oil of aerial parts of V. dentata was examined by GC-MS: it is constituted mainly by monoterpenes (57.16%), and of this the major components are 1,3,3-trimethyl-tricyclo[2.2.1.02,6] heptane (30.85%) and limonene (13.27%). Gram-positive and Gram-negative bacteria were sensitive to the essential oil. The essential oil had no antifungal activity. The hexane extract H1 presented antibacterial activity against seven strains (four Gram-positive and three Gram-negative); this extract had no antifungal activity. One bioactive compound, ent-kaur-16-en-19-oic acid 1, was isolated. In comparison with the essential oil and H1, the pure compound 1 showed the lowest MICs in the majority of the bacterial strains. Only the hexane partition (H2) of the methanol extract showed antifungal activity against all tested fungi.