The global trend of introducing second language learning, namely, English, in primary schools is increasing. In Saudi Arabia, where English has never been taught in primary schools, the government to implement English as a second language at the primary level in 2005; however, this generated controversy. Opposition to the learning of English has been based on religious, cultural, and educational arguments. The latter argument consists of claims that learning English at a young age might influence children's mother tongue development and influence their academic success. This paper investigates the impact of teaching English in Saudi primary schools on students' achievement in Arabic-language subjects. This quantitative research aims to inform the debate on second language learning in primary schools by studying children's examination results in the Arabic subject areas of grammar, reading, and writing. The sample consisted of primary school students from years 1 to 6 as well as year 6 students from the last year before (2004) and the first year after (2005) the introduction of English. Student results from four primary schools (two government schools and two private schools) were collected and analysed. This study found no indication of a positive or negative impact of learning English on students' achievement in Arabic subjects. However, private school students who studied English beginning in their first year of school had better results in the Arabic subjects that were the focus of this research.