Lanthanum selenide has been found to be a very good system in which to investigate the dependence of superconductivity on various material parameters, since in this system one can vary these parameters in a relatively independent manner. This investigation reports on the behavior of the superconducting transition temperature as a function of electron concentration and on measurements of some relevant electronic and lattice properties of the system. Crystallographic, Hall-effect, Pauli-susceptibility, and specific-heat measurements have been made and were found to be in accord with a model of lanthanum selenide as a free-electron-like metal. Using this model, the electron-concentration dependence of the transition temperature was calculated and found to be in excellent agreement with experiment over the whole experimentally measured range of 1 to 10A°K, corresponding to a variation of electron concentration between 0.8 and 5.4×1021 electrons/cc. © 1968 The American Physical Society.