The endangered Palos Verdes blue butterfly Glaucopsyche lygdamus palosverdesensis is endemic to the Palos Verdes Peninsula near Los Angeles, California, USA, where the butterfly or its sole larval foodplant, Astragalus trichopodus var. lonchus (Fabaceae), formerly occurred in 18 sites in association with a coastal sage scrub plant community. Even though the butterfly was recognized as endangered in 1980 by the US Fish and Wildlife Service, several of these sites were subsequently destroyed by housing or recreational developments. In addition, native vegetation at all remaining sites has been severely altered by fire control practices and the spread of weeds. Collectively, these actions have reduced numbers of A. trichopodus var. lonchus by more than 90% during the past 10 years and threaten to extirpate the Palos Verdes blue butterfly, if they have not already done so. Findings of field studies on the distribution and abundance of the butterfly and its larval foodplant, during the period 1979-1986, are summarized. No life stages of the butterfly have been observed since 1983, despite intensive annual searches. Immediate and large-scale rehabilitation of remaining habitat sites is necessary to increase numbers of the foodplant and, hopefully, the butterfly.