Hyphae and hyphal extracts from fungi isolated from inside healthy black spruce needles were assayed for their effect on spruce budworm (Choristoneura fumiferanaClem.) cell cultures and larvae. Isolates were from needles of four age-classes (current –3 years old) collected throughout a 4-month sampling period. Hyphal extracts from 21 of the 100 isolates tested were toxic to spruce budworm cell cultures and isolates from older needles (2 and 3 years old) negatively affected cells more frequently (p < 0.05) than isolates from younger needles (current and 1 year old). Hyphae from five isolates and hyphal extracts from seven increased mortality (p < 0.05) when fed to second instar larvae in a laboratory bioassay. Development was delayed and larval dry mass reduced when budworm were fed hyphae fromCryptocline abietinaPetrak (isolate number 69). Larval dry mass was reduced when budworm were fed the extract fromAureobasidium pullulons(de Bary) Am. (isolate 87) but increased when fedCryptocline abietina(isolates 40 and 80), a coelomycete (isolate 12), andHormonema dematioides(isolates 28, 63, and 96).Key words: endophytic fungi, budworm,Choristoneura fumiferana,Picea mariana, toxicity.