Cardiac valves and particularly, the left atrioventricular valve (LAVV) have long been considered passive structures. Nonetheless, there are more recent hypothesis that recognize this structure as a "living valve", with greater autonomy and dynamic function. Along these lines, some studies have concluded that the absence of contractile tissues in a valve, generates non-physiological undulations. In contrast, the presence of contractile tissue in the valve has been reported, reflecting a waving activation. Based on the above, the objective of the present study was to determine the presence of cardiac muscle fibers in the cusps of the LAVV. 12 cusps, 6 anterior and 6 posterior. Therefore, 7 adult (4) and lactating (3) bodies of different ages without cardiac pathologies were used. The samples belonged to the Faculty of Medicine of the Universidade de Ciencias da Saude de Alagos, Maceió, Brazil. The samples were treated with routine histological processing. Morphological findings at an increase of 4x showed muscle cells that were visible mainly from the atrial side in all the cusps, both immersed in the dense connective tissue of the fibrous ring and in loose connective tissue. Transverse striations were detected in all samples studied, when verifying the nature of the fibers with greater increase (100x), confirming the presence of cardiac striated muscle fibers in the LAVV. The results obtained contribute to the knowledge of the microstructure and contractile tissue of the LAVV cusps. Therefore it is relevant to further morphological knowledge of this valve, in order to build a solid foundation on the contractile microstructure in the different stages of the human development.