Undiluted l-phenylalanine has been cooled to 6K and its transmission spectrum obtained under terahertz radiation from a synchrotron source. Three distinct absorption bands are evident: at 1.37, 2.14, and 2.32THz. Each of these tracks to lower frequency ("redshifts") as the temperature is increased from 6 to 250K. The observed shifts are in the range of 0.1-0.2THz. The form of the temperature dependence is well accounted for by a Bose-Einstein model, from which the zero-temperature frequency of each mode and the characteristic temperature of the associated phonon bath may be estimated. At 6K a fourth band is evident, at 2.65THz. However, the depth of this, touching the noise floor, coupled with the increasing opacity of the sample with temperature for frequencies beyond 2.5THz, makes it difficult to track. The frequencies of all four modes are in good accord with and thus confirm a previous calculation.