The temperature dependence of the thermal diffusivity, α, of a lyotropic liquid crystal is investigated using the thermal lens technique in the range of 12–52 °C. The sample under study presents a calamitic-nematic phase between 15.0 and 47.2 °C. Below and above these transition temperatures it is in the isotropic phase (the phase below 15.0 °C is named reentrant-isotropic, I _RE ). The sample is aligned in a magnetic field before measurements, with the micelles parallel or perpendicular to the cuvette sidewalls, thus allowing the determination of the thermal diffusivity in both directions. The observed anisotropy in the thermal transport parameter is analyzed based on a model, which considers the geometry of the micelles and the order parameter S as a function of temperature. © 2003 American Institute of Physics.