Life habit: lichenicolous, on thallus of Diploicia canescens Thallus: not apparent, vegetative hyphae brownish, densely surrounding the hosts algae photobiont: absent Ascomata: very numerous and aggregated to form brown spots, sessile, 0.07-0.13 mm in diam., disc: black, convex, with rough surface epihymenium: brown, 2-5 µm thick, hyphal tips 4-5 µm wide hymenium: hyaline to pale brown, 25-35 µm tall; paraphysoids: branched and anastomosing; 2-3 µm wide; subhymenium: pale brown to brown, 20-35 µm thick asci: subglobose to clavate, 15-30 x 10-17 µm, usually 4-spored, lateral walls not thickened ascospores: hyaline, 1-septate, obovate, lower cell attenuated, smooth, 8-13 x 3.5-5 µm Pycnidia: not observed Chemical reactions: ascomata gels I+ red, KI+ blue, with minute KI+ ring in the tholus. Substrate and ecology: see host World distribution: Iberian Peninsula, Canary Islands, and North America Sonoran distribution: Baja California (Cerro Kenton adjacent the Pacific Ocean). Notes: Arthonia diploiciae is a comparatively aggressive species, which destroys the host thallus locally. Some variation is observed in this species: Materials from Ireland and Sardinia have slightly larger ascospores, and Hafellner (1995) found specimens with 8-spored asci. Arthonia aggregata, which occurs on Amandinea, could be a related species.