Twenty-five-year records of Ca Ⅱ H and K chromospheric emission fluxes measured in single lower main-sequence stars at Mount Wilson Observatory reveal surface magnetic activity cycles in one-third of the sample of roughly 100 stars. For those stars with cycles, we compare the ratio of the observed periods of the cycle of magnetic activity and axial rotation, P_(cyc)/P_(rot), to predictions available from stellar dynamo theory. Theoretical considerations suggest that the ratio is the observational equivalent of the stellar dynamo number, D. The stellar sample is comprised of two groups separated by their mean level of activity, < R_(HK)' > , and rotation, P_(rot): one group has high levels of average activity and fast rotation, while the other group has relatively low levels of activity and slower rotation. Both groups also occupy different regions on the diagram of X-ray flux versus stellar dynamo number. For the older group of stars (ages approx > 2 Gyr) which includes the current Sun, we find a statistically significant inverse relation between the intensity of the cycle, c = ΔR_(HK)'/ < R_(HK)' > , and the stellar dynamo number, empirically determined to be D ～ P_(cyc)/P_(rot)~(1.35_(-0.35)~(+0.65)).
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