Examples and data based on the host-parasite complex of the California red scale, Aonidiella aurantii (Maskell), and its parasites in untreated test plots in the citrus areas of southern California, not including the San Joaquin Valley and the desert areas, show the following: (1) Weather does not impose upper limits to host population increase. (2) In areas of favorable weather, certain parasites regulate host population densities at very low levels, other factors also being favorable. (3) The parasite Aphytis lingnanensis Compere is affected by weather differentially with respect to the host, and to varying degrees in different localities. The effects of field weather for each 2-week period of the year on all stages of this parasite individually as well as on population growth in held experimental cages are discussed. Adverse effects of weather on the parasite preclude successful population regulation of the host by the parasite at certain times or in particular areas but the range of the parasite is not restricted. (4) Another parasite, Comperiella bifasciata Howard, has its range restricted by weather and not by competition with other parasites to intermediate and interior citrus growing areas. It is never found along the coast. (5) Yet another parasite, Prospaltella perniciosi Tower, has its range restricted by competition and not by weather although it gives the appearance of being restricted by weather because it occurs only in coastal districts. (6) Examples are discussed of interactions between weather and dust, ants, or chemicals with parasites and hosts which result in various modifications of the abundance of the host and parasites. (7) Examples are discussed of the effect of weather on interactions between competing species of parasites and the host which have resulted in striking modifications of the previous distribution of certain parasites.