AbstractEvidence exists for a non-verbal capacity to apprehend number, in humans^1^ (including infants^2,3^) and in other primates^4-6^. Here we show that perceived numerosity is susceptible to adaptation, along with primary visual properties of a scene like colour, contrast, size and speed. Apparent numerosity was decreased by adapting to large numbers of dots and increased by adapting to small numbers, the effect depended entirely on the numerosity of the adapter, not on contrast, size, orientation or pixel density, and occurred with very low adapter contrasts. We suggest that numerosity is also an independent primary visual property, not reducible to others like spatial frequency or density of texture^7^.