Target probability has a well-known effect on detection times: Targets that occur with lower probability are detected more slowly than their higherprobability counterparts. A long-standing issue of interest is what causes this effect. In the two experiments of this study, we examined the possibility that the target probability effect has an attentional locus. We report two key findings that are consistent with this hypothesis. First, we observed a magnification of the effect when the attentional resources available for target detection were limited. Second, we also observed the complementary pattern: an attenuation of the effect when more attentional resources were available for detection. We propose that the target probability effect is caused by an asymmetry in the attentional demands made by targets that occur with different probabilities, with low-probability targets being more attentionally demanding than high-probability ones.