An upper bound is placed on a combination of $\mathrm{CPT}$- and Lorentz-violating quantities using data from a Penning-trap experiment with a single trapped electron. The experiment involves searching for diurnal variations in the electron anomaly frequency. The theoretical framework is a standard-model extension that violates these symmetries. The figure of merit for Lorentz and $\mathrm{CPT}$ symmetry in this context is bounded at 1.6 parts in ${10}^{21}$.

• Received 11 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.83.2116