A total of over 210 hr of previously unpublished differential photoelectric photometry of 21 Com (HD 108945, HR4766), obtained in five years, were analysed to search for periodicities. Of these, the 88 hr obtained in 1983, 1985, 1986 and 1989 were searched for, and show no indications of short-period variability. In 1988, over 116 hr of differential photoelectric photometry of 21 Com were obtained from six observing sites in North America, Europe and Asia in Strömgren b in order to look for rotational, as well as pulsational, periodicities. Most of the data were obtained during a three-week period of time in 1988 March-April, and nearly 6 hr of additional data were obtained in 1989 June. No statistically significant short-period variability was detected in the period range of 4.8 min to 2 hr in any of these data. The lack of detection of short-period variability contradicts the findings of numerous other authors, most recently those of Santagati et al. We have found a double-peaked lightcurve with a period of 2.00435±0.00080 d and a peak-to-peak amplitude of about 0.03 mag in b that we attribute to rotational variability. The time-base between our 1988 and 1989 data allows us to establish this accuracy for the period. We present arguments which rule out a single-wave lightcurve of half this period (namely 1.002 d).