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The progress of xylem formation in Norway spruce (Picea abies (L.) Karst.) was measured during one growing season in southern Finland. Stem radius was monitored continuously with band dendrometers, and the formation of new tracheids was determined by examination of small increment cores taken twice weekly. Tracheid production started in June and ceased in August. Xylem formation was fastest in early July, when 0.75–1.25 new tracheids were formed per day. The rate of xylem formation was significantly correlated with mean daily temperature. Synchronous fluctuations in tracheid and lumen diameters were observed at the same relative positions within each annual ring, but no relationship existed between the diameters and weather variables. The timing of changes in stem radius differed from the timing of actual xylem formation. Stem radius increased in April and May, and the fastest daily …