California phlebotomine sandflies carry at least seven protozoan species, including trypanosomes, hemogregarines, malaria and gregarines. This article discusses the arthropod-host-parasite complex, with infection foci centered around ground squirrel burrow systems in grasslands, chaparral and oak-pine woodlands. The complex illustrates several concepts of landscape epidemiology, insect bionomics and vector potential, and community dynamics. Transmission occurs both inside and outside the burrows and employs either inoculative or contaminative routes. Prevalence varies seasonally and yearly with parasites overwintering outside the flies. Natural infections were seldom harmful, but more infected flies died during transport. Some parasites interacted with each other. Parasite life cycles are adjusted to the short adult life span of the fly, while the earlier annual generations of sandflies may be more capable transmitters. INTRODUCTION This article concludes a 4-year study of California sandflies as hosts of protozoan parasites. It is a holistic view of an arthropod-parasitehost complex centered around colonial rodent burrows. It presents some new findings on the parasites and shows how the various populations are associated in a natural community. Seldom have so many parasites been associated with a specific host group in a particular geographic landscape. Chaniotis and Anderson (1968) found trypanosomes in wild California sandflies, and described their role as hosts of infectious agents. In other areas, sandflies carry human viral, bacterial and protozoan infections (Barnett, 1962). They transmit the leishmanias, causing a variety of diseases in man and animals (Adler, 1964; W.H.O., 1971). They also carry trypanosomes, harbor developing hemogregarines, monoxenous flagellates and gregarines, and are implicated as transmitters of reptile malaria. Our work in California has clarified many of these life cycles. MATERIALS AND METHODS Study methods for specific parasite groups were discussed in earlier publications (Anderson and Ayala, 1968; Ayala, 1970a-d; 197ta-c; Ayala and Lee, 1970; Ayala and McKay, 1971; Chaniotis and Anderson, 1968). More than 1600 sandflies and 1150 wild verteb;rates were examined for natural infections. Most animals were returned to Berkeley to study the parasites in natural and experimental hosts and in cultures. Infections were followed in laboratory-infected sandflies, and naturally infected flies were brought from the field to transmit their infections to laboratory-reared vertebrates. 1 Present address: University of Valle Medical School, International Center for Medical Research and Training, Apartado aereo 5390, Cali, Colombia,