Life habit: not lichenized Thallus: disrupting bark surface and making it paler photobiont: absent Ascomata:: ± round to lirellate, erumpent, 0.2-0.7 mm long, disc: black, ±flat, without pruina epihymenium: olivaceous brown, c. 10 µm thick; paraphysoidal tips: often with pigment caps, 2-3 µm wide, gel brown, pigment granular hymenium: hyaline, 25-35 µm tall; paraphysoids: hyaline, with 1-1.5 µm thick hyphae embedded in a gel; subhymenium: hyaline 10-20 µm thick asci: ovoid, 18-24 x 10-15 µm, 8-spored ascospores: persistently hyaline, 1-septate (not incised), broadly ovoid, 7-9 x 3-4 µm, without an epispore Pycnidia: c. 0.1 mm wide; wall: olivacous brown conidia: bacilliform, straight to slightly curved, 5-6 x 1 µm Chemical reactions: ascomatal gels I+ red, KI+ blue; ascus tholus with KI+ blue ring-structure. Substrate and ecology: on bark World and Sonoran distribution: known from southern California (Santa Monica Range) and Baja California Sur. Notes: The delimitation of this species requires further study of related taxa. Specimens with larger ascospores may belong to Arthonia galactites (known from Europe), which differs by K+ reddish pigments, or to A. celtidicola (with KI+ bluish epispore). According to Sundin (1999), the records of A. cytisi from North America are probably wrong. Arthonia cytisi s. str. is known from northern Italy and has narrow spores (13-16 x 4-5 µm), I+ persistently blue ascomatal gels, and KI+ bluish epispores.