A resonance lidar has made regular measurements of mesospheric potassium at the Arecibo Observatory (18.3¿N, 66.75¿W) since July 1999. Through July 2001 a total of 65 nights of density measurements were carried out, amounting to several nights each month over most of the period. The layer shows considerable nightly and seasonal variation: Spring and early summer nights had frequent sporadic layer activity; nightly variability in the layer width was large due to sporadic layer activity, but the seasonal variation was small; the altitude of the layer centroid was fairly constant throughout the year; and the column abundance showed semi-annual variation. These results are compared with a similar set of potassium measurements from a mid-latitude site, and the latitudinal implications are discussed.