This action-research study analyzed the effect of Content and Language Integrated Learning (CLIL) on the development of reading comprehension skills in English of 21 students of eleventh-grade in Colombia. A diagnostic test and the Survey of Reading Strategies were administered in class in order to establish the students’ level of reading comprehension in English and the reading strategies they used. A series of interventions to develop reading skills were carried out through instructional material based on the CLIL approach. The data was collected using qualitative instruments, such as the researcher’s field journal, an observation protocol and a focus group. Initially, students used very few or no reading comprehension strategies effectively. Moreover, they had poor performance in reading comprehension tests. After the implementation, the students demonstrated a more controlled and monitored use of their comprehension strategies that significantly influenced the development of their reading ability.