This study investigated the developmental and fossilized prepositional errors in Persian EFL learners' compositions at three levels of proficiency; participants were divided into lower-intermediate, upper-intermediate, and advanced levels. For each participant, four compositions were collected, and after identifying the prepositional errors for each level, their frequency and mean differences were compared across these levels to identify developmental and fossilized errors. Fifteen prepositional error types were identified, of which the most frequently made was the wrong selection of prepositions before nouns. Out of 15 error types, two were identified as developmental and 12 as fossilized. Findings indicate that most of the students committed fossilized errors, which shows the permanent retention of prepositional errors in their compositions. Accordingly, serious pedagogical consideration needs to be given to the teaching of prepositions to improve the writing competence of Iranian EFL learners. (Contains 13 tables.)