We present a method for acquiring ontological knowledge using search query logs. We first use query logs to identify important contexts associated with terms belonging to a semantic category; we then use these contexts to harvest new words belonging to this category. Our evaluation on selected categories indicates that the method works very well to help harvesting terms, achieving 85% to 95% accuracy in categorizing newly acquired terms. Categories and Subject Descriptors I.2.6, I.2.7 [Learning & Natural Language Processing]: Knowledge Acquisition, Text Analysis. General Terms Experimentation, Human Factors, Languages Keywords Named Entity, Ontology, Knowledge Acquisition, Query Logs.