Modern textbooks are valuable as a research object in applied linguistics because they usually present their content through multiple semiotic modalities and because in general they are discipline-specific genres. However, concerning the discipline of Biology, there are still few studies that exploit the interaction between verbal and visual languages in highschool textbooks and, up to the moment, no contrastive works on this topic have been published in Brazil. For this reason, the goal of the present study is to investigate the types of visual representations and the strategies that establish the interaction between pictures and verbal text used in two highschool-level Biology textbooks, a Brazilian and a North-American one. This study is based mainly on Kress and van Leeuwen s (1996, 2001) theory of multimodality and is inspired in Myers (1997) and Nascimento s (2002) previous works. This theoretical basis presupposes Halliday and Hasan s (1976) and Halliday s (1997) systemic- functional approach to text. The results of this investigation reveal that, among other mechanisms, lexical cohesion has an important role in the interaction between the verbal element in images and the verbal language of the corresponding texts and captions in both books. Among the differences found between these two books, there are the different visual representations concerning the same topic, the way pictures are referred to in the text, the role of the captions and the proportion of technical terms used in intermodal cohesion. These results show that there are intrageneric variations in the use of such resources that are probably due to the authors purposes. The findings may have important implications for the teaching of Biology and of English for specific purposes. They may enhance the awareness of educators and students about the need of taking the interaction between visual and verbal languages into consideration for the appropriate interpretation of texts and for the production of multimodal texts specific to each discursive community.