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In the canonical theory of stellar magnetic dynamo, the tachocline in partially convective stars serves to arrange small-scale fields, generated by stochastic movement of plasma into a coherent large-scale field. Mid-to-late-type M dwarfs, which are fully convective, show more magnetic activity than classical magnetic dynamo theory predicts. However, mid-to-late-type M dwarfs show tight correlations between rotation and magnetic activity, consistent with elements of classical dynamo theory. We use data from the Magellan Inamori Kyocera Echelle Spectrograph to detail the relation between CaiiH and K flux and rotation period for these low-mass stars. We measure$R_{\mathrm{HK}}^{\prime }$values for 53 spectroscopically identified M dwarfs selected from the MEarth survey; these stars span spectral classes from M5.0 to M3.5 and have rotation periods ranging from hours to months. We present the rotation–activity relationship as traced through these data. We find power-law and saturated regimes consistent to within 1σof previously published results and observe a mass dependence in$R_{\mathrm{HK}}^{\prime }$.