In two separate experiments in the field, healthy 80-year-old lodgepole pine (PinuscontortaDougl. var.latifoliaEngelm.) were inoculated with four blue-stain fungi that are associated with mountain pine beetle (DendroctonusponderosaeHopk.) and beetle-attacked trees. Inocula ofOphiostomaclavigerum(Robins.-Jeff. & Davids.) Harrington,O,huntii(Robins.-Jeff.) de Hoog & Scheffer,O.montium(Rumb.) von Arx, andO.minus(Hedge.) H. & P. Sydow were prepared on sawdust media. Inoculum was placed under the bark in two areas (8 × 5 cm each), one above the other, separated by a 15-cm space in which heat pulse velocity (HPV) sensors were installed. HPVs in the inoculated trees were monitored daily for 4 weeks. In sections inoculated withO.clavigerum, HPVs were significantly reduced 10 to 13 days after inoculation in both experiments. Sapwood under the inoculated area was visibly altered and colonized by the fungus. The other three fungi did not significantly reduce HPVs. These results proved the capability ofO.clavigerumto colonize functional sapwood and stop sap flow within a short period of time and reconfirmed the importance of this fungus in the mortality of trees attacked by mountain pine beetle.