The present invention features isolated DNA encoding allergens of Dermatophagoides (house dust mites) particularly of the species Dermatophagoides farinae and Dermatophagoides pteronyssinus, which are protein allergens or peptides which include at least one epitope of the protein allergen. In particular, the invention provides DNA encoding the major D. farinae allergens, Der f I and Der f II and DNA encoding the major D. pteronyssinus allergens, Der p I and Der p II. The present invention further relates to proteins and peptides encoded by the isolated D. farinae and D. pteronyssinus DNA, including proteins containing sequence polymorphisms. In addition, the proteins or peptides encoded by the isolated DNA, their use a diagnostic and therapeutic reagents and methods of diagnosing and treating sensitivity to house dust mite allergens, are disclosed.