BACKGROUND Polymorphic sequence substitutions in the major mite allergens can markedly affect immunoglobulin E binding and T cell responses, but there are few studies on environmental isolates from Dermatophagoides pteronyssinus and none for D. farinae. OBJECTIVE To determine the sequence variation of the group 1 and 2 allergens from environmental D. pteronyssinus and D. farinae. METHODS RNA from each species was isolated from homes in Bangkok and the sequence of Der p 1, Der p 2, Der f 1, and Der f 2 determined from cDNA produced by high fidelity polymerase chain reactions. RESULTS The enlarged data set revealed preferred amino acid substitutions in residues 19, 81, and 215 of Der p 1 as well as sporadic changes. Der p 2 showed frequent variations with clusters of amino acid substitutions, but the canonical Der p 2.0101 was not found in any of 17 sequences. Der f 2 showed variants with clusters of substitutions similar to Der p 2 but in different amino acid positions and without any structural concordance. Der f 1 in contrast to the other allergens had few amino acid sequence substitutions. CONCLUSIONS The sequence information on variants provides data important for the optimal design of allergen formulations and useful for the genetic engineering and structure-function analyses of the major allergens.