To date, much of the research that has used ultrasonic detectors as a tool to address questions on the spatial and temporal distribution of bat species activity have been limited by the lack of a reliable and quantifiable unit of activity, and a poor understanding of sampling intensity required to accurately assess site-specific activity levels. Here it is demonstrated that file size (i.e., bytes) of Anabat-recorded echolocation sequences of the little brown bat (Myotis lucifugus) was highly correlated with the number of calls, and was easily determined, and therefore represents a reliable and quantifiable unit of echolocation activity. Additionally, it is shown that accurate quantification of a site-specific magnitude of M. lucifugus activity may not be possible, even with a sampling intensity of up to 20 nights. As a result, ultrasonic monitoring studies must be designed to minimize the effects of the high variability in bat species activity at a site among nights.