A solid, crystalline fumigant (dazomet) with and without a supplemental copper compound was evaluated as an internal decay control treatment on Douglas-fir poles in two long-term field tests. Methylisothiocyanate (MITC), the decomposition product of dazomet, was used as a measure of effectiveness. MITC levels in the wood were above the threshold near the groundline application zone within 1 year after treatment. MITC levels above the groundline were much lower, suggesting that the treatment zone would need to be extended to produce protection in these higher zones. The addition of copper sulfate markedly increased MITC levels. Copper naphthenate was slightly less effective as a dazomet accelerant, but slightly better than dazomet alone. The results indicate that dazomet treatment remains at protective levels for 10 to 12 years. This range is well within the typical inspection cycle used by most North American utilities.