Mating behavior of the green chafer Anomala albopilosa sakishimana Nomura (Coleoptera: Scarabaeidae) was observed in Miyako Island, southwestern Japan from 28 to 29 May 2003. Females and males were present on patches of the beach naupaka, Scaevola sericea Vahl, throughout this time and showed clumped distribution within the patches. Males were observed to mount the backs of females for several hours during a day and successfully defend females from intruding males. Copulations took place only from 19:00 to 20:00 (JST) and most frequently at 19:40 (light intensity: 24 lx). Mean copulation duration was 18.1±2.6 min (mean±SD, N=16). After mating, both sexes remained on the same leaf to feed. When females guarded by males were collected before mating time and subsequently reared in the laboratory, about 65% (11/17) laid fertilized eggs. This indicated that most of the females mounted by males are not virgin. In laboratory observations, several females mated more than once on different days, indicating A. a. sakishimana females are polyandrous. Since mounted males successfully defended the females from intruding males in the laboratory, long-lasting mounts appear to comprise precopulatory mate-guarding behavior. This is the first record of precopulatory mate guarding in scarab beetles.