This study investigates the effects of topical knowledge on ESL (English as a Second Language) writing performance in the English Language Proficiency Index (LPI), a standardized English proficiency test used by many post-secondary institutions in western Canada. The participants were 50 students with different levels of English proficiency (basic, intermediate, and advanced) attending a Canadian college. Each student wrote two timed-impromptu essays: one responding to a prompt requiring general knowledge about university studies and the other pertaining to specific knowledge about federal politics. Results showed that students across three proficiency levels performed significantly better on the general topic than they did on the specific topic. The specific topic produced lower scores on content due to poor quality and development of ideas, implicit position taking, and a weak conclusion. Students also scored lower on organization and language on the knowledge-specific task because of weaker coherence and cohesion, shorter essays, more language errors, and less frequent use of academic words. Post-test interviews confirmed that participating students were challenged by the prompt that required specific topical knowledge. The study draws attention to the importance of developing appropriate prompts for ESL writing tests. (Contains 7 tables and 4 figures.)