The focus of this study was to explore the English language teachers' perceptions about the infusion of environmental issues into the teaching of English as a second language in lower secondary classrooms in Pakistan. The study was conducted in three private English medium schools in Karachi. A mixed methods approach utilizing both qualitative and quantitative methods was applied to investigate English teachers' perceptions about the infusion process. The methodology comprised of two different phases, namely Phase I and Phase II, respectively. Phase I was an exploratory phase, which involved one participant teacher teaching English in the lower secondary section. In this phase, qualitative data were mainly collected through interviews, classroom observations and document analysis. Based on the issues identified in Phase I, the Phase II of the study was conducted so as to expand the understanding by collecting data from a larger sample. Data in this phase were collected through a survey questionnaire from 23 lower secondary English language teachers from two schools. The major findings of the study indicated that environmental issues were infused into English teaching but most teachers perceived a lack of time due to their heavy workload to be a significant barrier to the infusion process. The findings also showed that English teachers have some basic understanding of environmental education but do not have a firm grasp of the whole discipline area. It was also found that despite the inclusion of environmental issues in the English textbooks the schools were using, the teachers put more emphasis on factual information, appreciation, and awareness, but did not insist on developing students' skills so as to make them take the necessary action to resolve environmental issues facing their communities. Of equal importance, the findings also indicated that teachers considered the clear environmental education policy and administrative support in schools as opportunities that would encourage them to incorporate more environmental themes into their teaching of English. With regard to the underlying teachers' perceptions about the infusion of environmental issues into their teaching of English, recommendations are made about how to help English teachers incorporate environmental issues into the teaching of English in a more effective way.