The elastic constants of a single crystal of the double hcp allotrope of neodymium have been measured over the temperature range 4.2–300 °K. The magnetic orderings which occur in neodymium near 7.5 and 19 °K are readily evident as cusps in the temperature dependences of some of the directly measured ultrasonic wave velocities, as well as in the associated elastic constants, and the character of the magnetic interactions is reflected in the differing effects on the various elastic constants. Comparison of the elastic constants of neodymium with those of seven other rare earths shows a trend with atomic number which is similar to trends which have been observed in other physical properties.