The treatment of molecular biology in thirty-four Spanish high school biology textbooks has been analysed using a check-list made up of twenty-three items. The study showed a tendency to confuse the genetic code with genetic information. The treatment of DNA transcription, regulation of gene expression and translation were presented as masses of factual information without necessarily facilitating the acquisition of an understanding of the main concepts. The biological meaning of terms such as "translation", "codified proteins" or "regulation of gene expression" were particularly unclear and poorly related to a wider understanding of cell biology. In conclusion, our data showed the need to improve text books by placing a greater emphasis on the understanding of basic concepts, rather than concentrating on factual detail, and by placing molecular biology in a broader context of cell and whole-organism biology. (Contains 4 tables and 3 figures.)