When the effectiveness of an instruction is based on the outcomes of the students at the end of the lessons, the heterogeneity of the classroom could not be ignored. Considering the fact that Malaysian classrooms consist of mixed-ability students, the Cambridge Baseline Study (CBS), which was conducted in 2013, prior to the education reformation, emphasised the use of Differentiated Instruction (DI). Since DI is relatively new to the Malaysian education system, little is known of the phenomenon of its implementation from various aspects, namely, its effectiveness, teachers' readiness, challenges faced, factors significant to its implementation, students' and teachers' perceptions, its aptness to Malaysian classrooms and needs of relevant stakeholders. In relation to this, the current study aims to investigate the factors influencing DI implementation in English language instruction. In addition, it also seeks to reveal if there are differences in the influencing factors between rural and urban schools. English language teachers from ten schools in Johor Bahru were selected for semi-structured interviews which revealed that teachers consider aspects related to Curriculum, Instruction, Knowledge of DI, School, Students, Time and Workload in the implementation of DI, and that there was no significant difference between the factors considered in rural and urban schools. Implications and future studies are discussed.