Abstract Absorption spectra of the bacteriochlorophyll a- protein from Prosthecochloris aestuarii were measured at temperatures from 2.9 to 300 K. Fourth and eight derivatives of the spectra were calculated from the digital data. From an analysis of 34 scans taken from 750 to 850 nm at 5 K, and 130 scans taken from 822 to 838 nm, we find evidence for nine peaks, six of which are probably 0-0 excitonic and three probably higher vibronic features. The major peaks are resolved in the derivative spectra to 300 K, and all shift with temperature by less than 1 nm compared to their 5 K positions, except for the 825 nm peak which shifts about 2 nm. The most prominent fourth derivative peak at 300 K shifts from 812.9 nm in the standard buffer solution to 814.1 nm in the cryogenic solution in which our low temperature measurements were made. We conclude that the conformation of the protein at 5 K is essentially the same as at 300 K.