--- - |2+ Abstract Objective We tested predictions about the structure and magnitude of method biases in single‐source personality trait assessments. We expected a large number of distinct biases that would parallel the observed structure of traits, at both facet and item levels. Method We analyzed multimethod ratings on the Estonian NEO Personality Inventory‐3 in a sample of 3,214 adults. By subtracting informant ratings from self‐reports, we eliminated true score variance and analyzed the size and structure of the residual method biases. We replicated analyses using data (N = 709) from the Czech Revised NEO Personality Inventory. Results The magnitude of method biases was consistent with predictions by McCrae (, Psychological Assessment). Factor analyses at the facet level showed a clear replication of the normative Five‐Factor Model structure in both samples. Item factor analyses within domains showed that facet‐level method biases mimicked the facet structure of the instrument. Conclusions Method biases apparently reflect implicit personality theory (IPT)—beliefs about how traits and trait indicators covary. We discuss the (collective) accuracy and possible origins of IPT. Because method biases limit the accuracy of single‐source assessments, we recommend assessments that combine information from two or more informants. - Journal of Personality, EarlyView.