The residual protective effect of sodium n-methyldithiocarbamate (NaMDC) fumigant was investigated in Douglas-fir timbers in a bridge in western Oregon using fungal colonization and levels of methylisothiocyanate (MITC; the primary fungitoxic breakdown product of NaMDC) as the measures of protection. MITC levels were above the presumed protective threshold 7 years after treatment and remained above that level in four of five timbers 12 years after treatment. These results differ from those found in round timbers and suggest that the combination of a protective-treated shell and the sawn surface resulted in a more prolonged protective period against renewed fungal attack. These results illustrate the benefits of NaMDC treatment on bridge timbers.