For the first time in Poland, seasonal dynamics and age structures of pyroglyphid dust mite populations were investigated in three beds from two dwellings in Sosnowiec (Upper Silesia). Simultaneously, the relative humidity and temperature of ambient indoor air was recorded during two years of the study (1984-1985). Generally, the increase of mite density was observed from April to November, and was related to the increase of indoor humidity. The mites were more abundant in the dwelling with central heating than in that with stove heating. Dermatophagoides farinae was the numerically dominant species (62.7% of the total mites), followed by D. pteronyssinus (28.8%). The analysis of the age population structure and its dynamics showed the marked differences between particular pyroglyphid mite species and the beds examined. In total, D. farinae populations showed the dominance of immature stages, whereas in populations of D. pteronyssinus and E. maynei, the dominants were adult mites.