In the modern educational process, teachers adapt lessons to the student's individual abilities, prior knowledge, interests and social skills. The present study, whose aim is to examine the inclusion of differentiation in literacy lessons in the first and second grades of Slovenian primary schools, was carried out on a smaller scale (N = 79). It aims to determine the views of first- and second-grade primary school teachers on differentiation, and to find out how differentiation takes place in literacy lessons and which factors affect student performance in class. The results show that the inclusion of differentiation in literacy lessons is present in the entire initial literacy teaching process, and that Slovenian teachers undergo regular training and professional development courses in this area, thus contributing to the educational success of students and affecting their personal growth and development. Teachers estimate that the intertwining of individual abilities, family environment, engagement in co-curricular activities and well-developed social skills represents a key factor in student success. It was expected that internal differentiation would be more effective than external differentiation. Surprisingly, however, teachers report that heterogeneous groups do not enhance educational efficiency.