This study examines errors in a corpus of 120 essays produced by 86 adult Turkish learners, who were beginners in their language proficiency in Cukurova University. The corpus was examined to identify and classify written errors in terms of the possible sources of errors. Errors were classified in accordance with two major categories: interlingual errors and intralingual errors, and some sub-categories were identified. It has been found that most written errors students produce result from the interlingual errors indicating interference of the first language. Some suggestions have been made in the treatment of errors. (C) 2010 Elsevier Ltd. All rights reserved.