The sound attenuation due to trees has been measured (under atmospheric conditions requiring no corrections) at about twenty locations, representing four selected types of woods. A constant excess attenuation of 7 dB/100 ft was found at frequencies below 2000 cps, in disagreement with previous workers. With the sound source in the open at the edge of the woods, effects due to change of medium were observed, viz. an increase of sound-pressure level in the woods and a distinctive S-shaped curve of attenuation vs frequency. This is suggestive of resonant absorption, although a theory developed according to such a hypothesis, while having the correct frequency dependence, fails to predict the magnitude of the observed results by a factor of 103. Transverse modes of vibration of the branches were shown to be the oscillators active in the frequency range of interest, 250 cps in the lower branches to about 1100 cps near the top of the tree; indications were that trunk resonances might also be important.