Using a letter search task, we tested a new postulate of the extended featureintegrationtheory of attention (Treisman & Gormican, 1988), namely the hypothesis of acontinuum of attentive processing. This hypothesis, which replaces the old dichotomy of astrict parallel vs. serial stage, implies that groups of items can be scanned in parallel withgroup size depending on target discriminability. The prediction that a variation of group sizeleads to varying slopes of the reaction time vs. display size functions was found to beconsistent with the data. More particularly, the data lend support to a general, formal groupprocessingmodel of attention, which was tested quantitatively. In addition, the resultsindicate that in visual search the relation between RT and target discriminability obeys asimple inverse power law (Jacobs, 1991 a & b).