This investigation compared readability formulae to publishers' identified reading levels in science-based elementary readers. Nine well-established readability indices were calculated and comparisons were made with the publishers' identified grade designations and between different genres of text. Results revealed considerable variance among the 9 formulae. All formulae tended to inflate readability calculations for nonfiction science-based text, whereas fiction science-based text was more closely aligned to the publishers' grade levels. Implications are discussed for elementary teachers' awareness of readability variances in science-based resources, and the professional learning that is required to support the use of elementary readers, including understanding the limitations of using common readability metrics. (Contains 12 tables and 1 figure.)