The seasonal dynamics of house dust mites,Dermatophagoidesspp., was investigated for 19 mo in southern Calif. In Orange Co. houses sampled,D. pteronyssinus(Trouessart) was generally more abundant from July–Nov. or Dec., whileD. farinaeHughes was more prevalent from Aug.–Dec. or early spring. Both species were less abundant from late spring to ca. July. In Riverside houses, both species were generally more abundant from late summer to early fall (1975), and from early spring to early summer (1976), with lower numbers present from June–July (both years). Monthly fluctuations of each species generally corresponded to climatic changes, particularly RH, with increased populations occurring 1–2 mo after conditions approached or became optimum forD. farinaeorD. pteronyssinus, while populations declined when RH reached or fell below critical levels for the respective species.