In Sweden, as in many other countries, there are major focuses on knowledge assessment and accountability in school. It has been decided that National Tests will be conducted in biology, physics and chemistry in grade 6 from 2013. According to the Swedish national curriculum, language and communication skills are important in all school subjects. The National Tests are expected to test the skills specified in the curricula, including the ability to communicate scientific knowledge. However, including the use of communication skills in a science test may prevent some students, such as students who do not have Swedish as their first language, from properly displaying their knowledge in science. The results of the national tests in science also show that students with Swedish as a second language perform lower in the tests than students with Swedish as their first language. Based on these results the purpose of this study is to investigate the difficulties of students with Swedish as a second language in relation to the part of the national test in science that assess communication skills. The study also investigates whether students' first language can be a resource in verbal conversation about the test questions. The study is based on test scores and interviews with 15 students with Arabic as their first language. Results from this study show that students' first language are used as a resource in the discussions and then primarily to get difficult words and concepts explained. Furthermore, the results indicate that the authentic (or realistic) questions contained in the tests, are likely to make the students' background important for their test performance, since similarcontents are not necessarily part of the teaching of science subjects. Most of the questions are also presented in an ”everyday context”, while the answers expected include scientific concepts and reasoning. Therefore the requirements and expectations become unclear to the students. Although these difficulties with authentic questions are not specific to students with Swedish as a second language, there is a risk that the difficulties have greater impact on this particular group of students. Thus, this can contribute to these students perform lower in the National Tests in science subjects, compared to students with Swedish as their first language.Inklusive 2 bilagor.