Chromospheric Ca II H and K fluxes have been measured in a sample of similar to 100 stars on or neat the lower main sequence at Mount Wilson Observatory. Observations were made several times a week and span more than 10 years. Within an observing season, many stars show periodic variations due to rotation. Thirty-six of the stars have highly significant periods in at least five seasons. We compute the range in the observed period, Delta P, and suggest that it is a measure of, and a lower limit to, the surface differential rotation. Several physical and selection effects can affect the measured Delta P value. An analysis of the cumulative variance distribution at various timescales, however, demonstrates that Ca II variations due to active region growth and decay are generally of longer period and smaller amplitude than those due to rotation. We argue that other effects (e.g., multiple active regions, latitude bands) either are small or primarily act to reduce the measured Delta P relative to its true value. Including results for the Sun, we find that Delta P depends on the mean seasonal rotation period [P] such that Delta P proportional to [P](1.3+0.1), independent of mass. We briefly discuss this result in the context of dynamo models, and other observations of surface differential rotation and active region structure.
