Bacteriochlorophylls-d (2) have been isolated from Chlorobium vibrioforme forma thiosulfatophilum, a green sulphur bacterium. Treatment of the bacteriochlorophylls with sulphuric acid in methanol affords the methyl bacteriopheophorbides (1) which are separated by high performance liquid chromatography (h.p.l.c.) into six major homologous fractions differing in the nature of 4-(ethyl, n-propyl, isobutyl) and 5-(methyl, ethyl) substituents. Using a combination of h.p.l.c., n.m.r., and X-ray crystallography, the 4-ethyl and 4-n-propyl homologues (1a), (1b), (lc), and (1f) are shown to have the (R) absolute stereochemistry at the 2-(1-hydroxyethyl) substituent while the 4-isobutyl compounds (lc) and (1g) are shown to have the opposite (S) chirality at this position. Two new methyl bacteriopheophorbides-d bearing a 4-neopentyl substituent (1d) and (1h) are isolated and characterized; these appear to be the first natural products with a neopentyl substituent to be isolated and are also assigned the (S) absolute stereochemistry at the 2-position on the basis of n.m.r., h.p.l.c., and X-ray structural evidence. The methyl bacteriopheophorbides-d from a new strain of green sulphur bacteria [Chlorobium vibrioforme(B1-20)] have also been studied; initially, these bacteria, which possess an abnormally short wavelength absorption in living cells, appeared to be producing major amounts of only two methyl bacteriopheophorbides-d (la) and (1e), but after they had grown continually for more than a year, the pigment composition began to contain larger quantities of more highly methylated homologues. The bacteriopheophorbides with R⁴= Et and R⁴= Buⁱ again had the (R) and (S) absolute stereochemistries, respectively, at the 2-position, but the chirality at the 2-(1-hydroxyethyl) group in the homologues with R⁴= Prⁿ is shown to be a mixture of both (R) and (S), this situation resembling that in the bacteriopheophorbides-c.