The ascomycete Colletotrichum gloeosporioides was tested for biological control of Arceuthobium tsugense on western hemlock (Tsuga heterophylla) in British Columbia (BC), Canada. A field trial was conducted near Nanaimo, BC, using C. gloeosporioides isolate PFC 2415 in three treatments applied in late August 2002. The treatments consisted of C. gloeosporioides formulated with ‘Stabileze’ (incorporation of fungi in a water-absorbent starch matrix with oil and sucrose, then granulating the matrix with hydrated silica) which were sprayed on (1) intact mistletoe swellings, (2) swellings on which all mistletoe shoots had been cut at 0.5 cm from the base of the shoot and (3) C. gloeosporioides formulated in a sucrose and gelatin preparation sprayed on intact mistletoe swellings. The ultimate goal of this research was to substantially reduce the ability of the treated dwarf mistletoe plants to produce seed inoculum. The ‘Stabileze’ and sucrose–gelatin treatments formulated with C. gloeosporioides reduced the current berry crop by 36.8 and 40.5%, respectively (p < 0.05). While the results for shoots appeared promising, heavy background infection and/or secondary infection, especially on controls, limited the ability to detect clear treatment effects. Careful culturing from various live and dead host tissues showed that C. gloeosporioides was unable to invade and kill the mistletoe endophytic system within the living xylem and phloem of the host.