Prior knowledge activation is a crucial component of reading comprehension. Previous studies have examined students' prompted (or solicited) purposeful knowledge activation, which occurs when the explicit goal is to activate knowledge, as well as ancillary knowledge activation, which is when students indirectly use their prior knowledge to fill in gaps in the text, form an opinion, or question the author. However, little is known regarding elementary students' unprompted (or unsolicited) purposeful and ancillary activation of prior knowledge while reading grade-level texts. The purpose of the current study was to (a) examine differences between third- and fifth-grade students on their use of purposeful and ancillary prior knowledge activation when reading grade-level social studies and science texts and (b) determine how students' prior knowledge activation relates to their reading outcomes and reader profiles. Participants were 25 third-grade and 27 fifth-grade students from an urban school system in the southeastern United States. Participants were asked to think aloud as they read grade-level texts. Utterances were transcribed and coded according to the type of knowledge activation. Although repeated measures ANOVA revealed no significant differences in purposeful or ancillary activation across texts or between grade levels, the way in which students utilized these two types of prior knowledge activation (i.e., purposeful and ancillary) differed as a function of their reader profile. To uncover these differences, we took a case study approach to further explicate the complex relations between prior knowledge activation, other reading behaviors, and reading outcomes.