Good candidates for naturally occurring variability in circadian rhythms may be subterranean herbivores, since they are not normally subjected to entraining light stimulation. To test this possibility, we selected the blind mole rat Spalax ehrenbergi superspecies in Israel and tested it in short- and long-term experiments. Short-term experiments showed that the animals exhibited three patterns of activity: a regular circadian rhythm (26.6%), an altered circadian rhythm (shorter or longer than normal, 53.1%), and an arrhythmic pattern (20.3%). A long-term experiment showed that the arrhythmic pattern indeed reflected a genuine arrhythmic genotype. The mole rats were found to be active less than 25% of the day and exhibited a multiphasic mode of activity, both diurnally and nocturnally. The number of activity periods and the level of activity were negatively correlated: Animals that exhibited a high level of activity per unit of time showed low numbers of activity periods, while animals that exhibited a lower level of activity showed higher numbers.