Typical natural visual scenes contain many objects, which need to be segregated from each other and from the background. Present theories subdivide the processes responsible for this segregation into a pre-attentive and attentive system(1,2). The pre-attentive system segregates image regions that ＇pop out＇ rapidly and in parallel across the visual field. In the primary visual cortex, responses to pre-attentively selected image regions are enhanced(3-5). When objects do not segregate automatically from the rest of the image, the time-consuming attentive system is recruited. Here we investigate whether attentive selection is also associated with a modulation of firing rates in area V1 of the brain in monkeys trained to perform a curve-tracing task(6,7). Neuronal responses to the various segments of a target curve were simultaneously enhanced relative to responses evoked by a distracter curve, even if the two curves crossed each other. This indicates that object-based attention is associated with a response enhancement at the earliest level of the visual cortical processing hierarchy.