Bamboo is a highly anisotropic natural material widely used in structural applica- tions. A transversely isotropic constitutive equation is proposed here as the simplest law able to capture main anisotropic features of bamboo. Diametric compression tests were undertaken to determine the circumferential Young's modulus of two bamboo species based on the transversely isotropic model and using a simple formula of elasticity, validated with finite element analyses considering true and simplified geometries. Results showed that the use of idealized geometries and simple formulas provided a good estimation of the circumferential modulus. Load-deformation curves displayed a softening behavior which was approximated with two straight lines. Using the slope of the first line, the circum- ferential moduli were found to be 485 ± 172 MPa and 1685 ± 132 MPa for Guadua angustifolia and Phyllostachys pubescens, respectively. For Guadua angustifolia the circumferential modulus showed a significant increase with height, registering values of 651 ± 153 MPa for the top, 465 ± 126 MPa for the middle and 344 ± 98 MPa for the bottom parts of the culm. For Phyllostachys pubescens the circumferen- tial modulus was independent of the length of the specimen. Results confirm the significant anisotro- pic behavior of bamboo, that should be considered