The influence of air-fall tephra (ash) from the 1980 Mount St. Helens eruptions on decomposition of Douglas-fir (Pseudotsugamenziesii(Mirb.) Franco) needles was studied in a field experiment at the Cedar River watershed near Seattle, Wash. Ash that fell on Yakima, Wash., was transported to the site and placed on 5 × 5 m plots in a 50-year-old Douglas-fir stand at depths of 5 and 20 cm. Objectives of the study were to determine: (i) decomposition rates of Douglas-fir needles on the ash surface, buried under ash, and in an untreated control; (ii) the effect of ash depth on decomposition rates; and (iii) the influence of ash on forest floor temperature and moisture. Yakima ash had considerable influence on forest floor temperature and moisture and Douglas-fir needle decomposition rates. Needles buried under 5 cm of ash had increased rates of decomposition after 2 years (k = 0.595/year) relative to control needles (k = 0.347/year), while those buried under 20 cm of ash had a similar rate of decomposition to control needles (k = 0.349/year), perhaps owing to ash compaction. Those on the ash surface had decreased rates of decomposition relative to buried needles. These results are similar to results found in the ash fall zone near Mount St. Helens, where needles located on top of ash decomposed slower than buried needles. Increased decomposition of tephra-affected forest floors, relative to unaffected areas, may have facilitated plant regrowth by increasing the availability of limiting nutrients. Ash created a more favorable temperature environment for decomposition beneath the ash with the forest floor under ash being cooler in summer and warmer in the cooler months. Forest floor moisture was reduced under the ash but did not appear to be limiting to decomposition.