Large aggregations of solitary bees have been reported from natural and managed landscapes where abundant nesting habitat and floral resources co-occur. Here we describe an unusually large aggregation of the bee Anthophora (Melea) bomboides Kirby, 1838 (Hymenoptera: Apidae) nesting in a coastal bluff in San Juan County, Washington State. Nest densities as high as 630/m² were recorded along a steep sandy embankment measuring 1.0 km long with a mean height of 9.3 m. The population estimate of 133,492 nesting females exceeds previously reported occurrences for this species by two orders of magnitude. Floral resources supporting this aggregation over three years of observation included dense flowering shrubs covering 16 ha of abandoned farmland immediately adjacent to the bluff. We also report an additional 31 species of bees and wasps collected at the site and discuss recently adopted conservation measures to protect the site's floral resources from conifer encroachment.