Colonies amphigenous, punctiform to effused, black when sterile, grey when sporulating. Mycelium partly immersed and partly superficial, pale brown, branched and anastomosing, sometimes aggregated into a pseudoparenchymatous layer, smooth, septate, 2-4 µm thick. Setae sterile, originating from dark brown, thick-walled, swollen cells of the superficial mycelium, simple, erect, straight or flexuous, usually smooth but sometimes minutely verruculose, septate, dark brown, paler towards the apex, up to 330 µm high, 5-7.5 µm wide at the base, gradually tapering to a width of 1.5-3 µm at the apex. Conidiogenous cells polyblastic, apparently percurrent, discrete, obclavate to lageniform, subhyaline, 7-12 µm high, 3-4 µm wide. Conidia acrogenoss, hyaline but adhering together at the base of setae to form a greyish-white mass, slightly curved or falcate, smooth, non-septate, 20-24 (-26) x 1.5-2 (-2.25) µm.

On Myrsine chathamica F. Muell.

Coloniae amphigenae, punctiformes vel effusee, nigri vel grisei. Mycelium partim intermum et extermum, ex hyphis pallide-brunneis, ramosis et anastomosantibus, laevibus, septatis, 2-4 µm crassis. Setae steriles, simplices, erectae, rectae vel flexuosae, plerumque laeves, septatae, atro-brunneae, apice versus pallidiores, usque ad 330 µm altae, ad basem 5-7.5 µm latae, ad apicem 1.5-3 µm latae. Cellulae conidiogenae polyblasticae, percurrentes visu, discretae, obclavatae vel lageniformes, subhyalinae, 7-12 µm altae, 3-4 µm latae. Conidia acrogenosa, hyalina, adherentia, leniter curvata vel falcata, laevia, eseptata, 20-24 (-26) x 1.5-2 (-2.25) µm. In foliis dejectis myrsinaceae speciei Myrsinae chathamicae F. Muell. Holotypus PDD 49391.

The specific epithet refers to the type locality, Chatham Islands.
Sutton (1980) provided a key to eight species of Circinotrichum, and since then C. ponmudiense Varghese & V. G. Rao (1978), C. britanicum Kirk (1981), C. flagelliforme Zucconi & Onofri (1986), and C. cochinense V.G. Rao & Varghese (1988) have been described. C. chathamiensis can be distinguished from other species of Circinotrichum by the size and shape of conidia, and by the straight, sterile setae.

Specimen examined: New Zealand, Chatham Islands, Rekohu, Taiko Camp, 8.III.1983, E. H. C. McKenzie (PDD 49391 - holotype).