An adjustable sampling tube construction for a duct-mounted smoke detector includes telescopic tube sections which can be fixed in overall length by a screw extending through an outer tube to fix an inner tube with respect to the outer tube. The inner tube includes air sampling holes spaced apart along its length and the outer tube includes a plurality of spaced apart through holes having a same spacing to register with the sampling holes of the inner tube. The inner tube is telescopically positionable within the outer tube to register the sampling holes with the through holes to provide air sampling locations throughout a length of the combined inner and outer tube assembly.