AbstractWe have recently suggested that numerosity is a primary sensory attribute, showing that it is strongly susceptible to adaptation. Here we use the Method of Single Stimuli (MSS) to show that observers can extract a running average of the numerosity of a succession of stimuli and hold it in mind for use as a standard of comparison for subsequent stimuli. Accuracy and precision of judgments are high and not reduced by potentially misleading variables like texture density or display area.