It is commonly assumed that certain features are so elementary to the visual system that they require no attentional resources to be perceived. Such ’preattentive’ features are traditionally identified by visual search performance(1-3), in which the reaction time for detecting a feature difference against a set of distracter items does not increase with the number of distracters. This suggests an unlimited capacity for the perception of such features. We provide evidence to the contrary, demonstrating that detection of differences in a simple feature such as orientation is severely impaired by additionally imposing an attentionally demanding rapid serial visual presentation task involving letter identification. The same visual stimuli exhibit non-increasing reaction time versus set-size functions. These results demonstrate that attention can be critical even for the detection of so-called ’preattentive’ features.