An in situ core sampling method was used to obtain absolute population density estimates for the mite, Dermatophagoides pteronyssinus Trouessart (Acari: Pyroglyphidae) in the mattress habitat. At the 95% fiducial limit the numbers of D. pteronyssinus in the top 1.5 cm of substrata were 8,200-26,800 mites m(-2). Mite densities that were calculated from vacuum samples of the same habitat underestimated the abundance of D. pteronyssinus by more than three orders of magnitude. Vacuuming did not detect all mite species inhabiting the mattress, and eggs and immature mites were under-represented in vacuum samples. The importance of absolute population estimates versus relative estimates of abundance and population density indices is discussed, and a number of ways in which the core sampling approach may contribute to future house dust mite research are suggested.