We studied the behavioral ecology of Plathemis lydia Drury (Odonata: Anisoptera) in central coastal California during summer 1983. Oviposition peaked in the early afternoon, was fairly synchronous, and varied spatially, with females avoiding a sandbar and preferring the deepest section of the pond. Only 79% of males known to be alive visited the pond on any particular day. Those that did come were territorial, nonterritorial poachers, or often both within the same day. Territories were always defended individually and individual males were territorial for only a few hours in any one day, even though mating success was considerably higher for territory holders. Daily mating success of males was estimated based on the time, place and duration of territorial behavior. Estimated daily mating success correlated significantly with wing condition and body length. The components of mating success we measured all correlated with one another; thus, we found no evidence that tradeoffs occurred either among components of daily mating success or between estimated daily mating success and survivorship. INTRODUCTION Territoriality in odonates has been known for nearly a century (see references in Jacobs, 1955) and has now been documented in a variety of both dragonflies (subfamily Anisoptera) and damselflies (Zygoptera) (e.g., Waage, 1979a; Parr, 1980; Sherman 1983a). Here we describe patterns of territoriality and mating in the white-tailed skimmer Plathemis lydia Drury. This species has previously been studied by Jacobs (1955) in Indiana and Campanella and Wolf (1974) in New York. Our goals are to describe (1) patterns of male territoriality; (2) mating patterns of males and females, including correlates of mating success in males, and (3) contrasts between our population and previously studied populations of Plathemis lydia. Plathemis lydia is a medium-sized, sexually dimorphic dragonfly occupying pond habitats in the United States and southern Canada (Needham and Westfall, 1955; Campanella and Wolf, 1974). Each day during the breeding season males go to ponds, set up territories, and compete for access to females which visit ponds to copulate and oviposit. An arriving female is usually pursued by one or more males attempting to seize her and copulate. Following successful copulations a female oviposits by hovering just above the surface while dipping her abdomen into the water to wash off eggs, which are laid at the rate of about 23 eggs per second (McVey, 1984). Meanwhile, the male mate hovers approximately 0.5 m above the female guarding heragainst intruding males who themselves try to copulate with the female (noncontact guarding, Parker, 'Present address: Department of Zoology, University of California, Berkeley 94720.