The aim of the study was to examine the lexical errors made by EFL students. The technique for eliciting information employed was an achievement test. A sample of 30 Saudi female students was asked to write essays in English that were assessed by the researcher. The students were all majoring in English in the third year at King Khalid University. James (1998) taxonomy was selected as the most comprehensive framework for the analysis of the lexical errors in the students' writing. A total of 137 lexical errors were identified and analysed. These errors were divided into formal 117 (85.40) and semantic 20 (14.60). Formal mis- selection 54 (39.42) was the most frequent major category of lexical formal errors while mis-formation 15 (10.95) was the least frequent one. Confusion of sense relations 14 (10.22) was the most frequent among lexical semantic errors. At the individual level of lexical formal errors, the most problematic words for students were the vowel based types 24 (17.52) and borrowing and blending were not problematic at all. At the individual level of lexical semantic errors, the most problematic words for students were near synonyms 8 (5.84) and the least problematic words were general terms for specific ones and overtly specific terms 1 (0.73). Pedagogical implications for teaching vocabulary to EFL learners and recommendations for areas for further research were suggested.