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, DNA encoding two major D. farinae allergens, Der f I and Der f II and DNA encoding a D. pteronyssinus allergen, Der p I. In addition, the proteins or peptides encoded by the isolated DNA, their use as diagnostic and therapeutic reagents and methods of diagnosing and treating sensitivity to house dust mite allergens.