The fourth-grade 2019 National Assessment of Educational Progress (NAEP) Reading assessment shows that 34% of the nation's students perform below the "NAEP Basic" level. However, because there is no achievement-level description for below Basic, educators and policymakers lack information on the nature of the reading difficulties that these students face. To help fill this gap, we analyze data from the 2018 NAEP Oral Reading Fluency study. We find that, compared with students who perform at the "NAEP Basic" level and above, students who perform below "NAEP Basic" level are much more likely to have poor oral reading fluency and word reading skills.