A synodic monthly rhythm of spontaneous activity in the Mongolian gerbil,Meriones unguiculatusis reported. Gerbil activity, plotted as deviation from a seasonal daily mean computed for each day of the synodic month for the four seasons separately, generally reaches maxima shortly after new moon and full moon. Minima occur around last quarter and several days before full moon. This pattern persists for the summer, fall and spring cycles, whereas the winter cycle is primarily a mirror image of the former three. Similarity is present in the cycle form and phase relationship (or an inversion of it) among the gerbil, hamster and mealworm. This similarity and also the extraordinary phase‐synchronization of the gerbil cycle from season to season suggests strongly that all organisms were influenced by a subtle factor or factors and that this factor(s) effected the varying degrees of spontaneous activity.