Announced herein is the discovery of a nesting site of Lithurgus (Lithurgus) chrysurus Fonscolombe (Megachilidae: Megachilinae: Lithurgini), an adventive solitary bee of Western Palearctic origin that had first been detected in North America in the mid-1970's and thought to have become extinct locally since then. The occurrence of the new nesting site in eastern Pennsylvania, close to the point of first discovery in New Jersey, suggests its persistent but low presence over the past 35 years. Several previous papers offered insight into nests of this species, which bores into dead wood to nest, and described its larva and pupa. The current investigation reveals new information concerning nest architecture, nest closure, its cocoon structure and functions, and anatomy of its mature larva. Its larva is compared with that of L. (Lithurgopsis) apicalis Cresson (a congener in another subgenus) and with that of Trachusa larreae (Cockerell) with consideration of how body vestiture and anatomy function to permit mature megachilid larvae to move in the confines of their brood cells.