Grammar is an important aspect of language learning and is thus of concern in language teaching as well. This study sets out to investigate the kinds of grammatical mistakes Swedish secondary school pupils make in their written production of English and the possible causes behind these different mistakes. Forty essays from pupils in year nine have been used to study the phenomenon of frequently occurring mistakes and their causes, and the salient mistakes have been quantatively and qualitatively accounted for in the study.      Common mistakes found in the material were subject-verb concord mistakes, mistakes involving plural nouns, incorrect article usage as well as problems using prepositions correctly. Interlingual transfer, i.e. negative transfer from the pupils’ L1, proved to be one significant factor causing mistakes in the material. However, other causes could be traced as well, such as intralingual transfer for example.      Consequently, the findings of this study point in the direction of areas which need specific attention in English L2 learning in Swedish classrooms. Thus, the study contributes with valuable insights for English teacher and teacher trainees to consider in their future practice.