The low-temperature behavior of a natural kalsilite (ideal formula KAlSiO4) with P31c symmetry has been investigated by in situ single-crystal diffraction. A series of intensity data collections and structural refinements have been performed at 298, 250, 200, 150, and 100 K on decreasing temperature, and 175, 225, and 275 K on increasing T. The variations of the unit-cell parameters of kalsilite as a function of T are continuous, and show no evidence of any phase transitions or thermo-elastic anomalies in this temperature range. An expansion is observed along  with decreasing temperature. The axial and volume thermal expansion coefficients (αj = lj−1·∂lj/∂T, αV = V−1·∂V/∂T) between 298 and 100 K, calculated by weighted linear regression through the data points, are αa = αb = 1.30(6)·10−5, αc = −1.5(1)·10−5, αV = 1.1(2)·10−5 K−1. The main structural change on decreasing temperature is a cooperative anti-rotation of tetrahedra forming the six-membered rings lying parallel to (0001). This tetrahedral rotation is coupled with a change in the distances between the extra-framework cations and the framework O atoms. A small decrease in the tetrahedral tilts perpendicular to  is responsible for the negative thermal expansion along ; the implications of these mechanisms for thermal expansion in nephelines and kalsilites are discussed.