The protein, lipid, and fatty acid composition of the thylakoid membrane system of western hemlock (Tsuga heterophylla (Rafn.) Sarg.) chloroplasts was determined. Linear gradient slab gel electrophoresis of SDS-solubilized membrane protein preparations from the thylakoid system and its photosystem I (D144) and II (D10) subfractions resolved 32 protein bands. Density analysis of electrophoretic patterns accompanied by molecular weight determinations distinguished complex I at 63 kilodaltons and complex II at 23 kilodaltons.Sequential extraction of the thylakoid pellet in acetone - ethyl ether (4:1, v/v) and chloroform–methanol (2:1, v/v) followed by gel filtration on lipophilic Sephadex LH-20 provided two major lipid fractions. Qualitative thin-layer chromatography using lipid standards and colorimetric assays revealed the presence of monogalactosyl diglyceride, digalactosyl diglyceride, sulphoquinovosyl diglyceride, phosphatidyl glycerol, phosphatidyl choline, and phosphatidyl inositol. The concentration of each glycerolipid in micromolars per gram fresh weight of needle tissue was 11.16, 9.90, 6.18, 5.25, 3.16, and 2.32, respectively.The fatty acid contingent of each glycerolipid was determined by gas–liquid chromatography using 15% HI–EFF–IBP on Chromosorb W (100–200 mesh). Trimethyl(α, α, α-trifloro-m-tolyl)ammonium hydroxide was used as the 'on-column' active methylating agent. The following fatty acids were present at detectable concentrations in each of the glycerolipids: palmitic (16:0), palmitoleic (16:1), stearic (18:0), oleic (18:1), linoleic (18:2), linolenic (18:3), and 11-eiconsenoic (20:1). The major fatty acids of the phospholipids were 16:0 and 18:1, while the predominant fatty acids of the glycolipids were 18:3, 18:1, and 16:0.Western hemlock thylakoid membrane protein patterns appeared remarkably similar to those demonstrated in numerous plant and algal systems. On the other hand, thylakoid glycerolipids and their respective fatty acids, while qualitatively similar, revealed significant quantitative differences from values reported for herbaceous species.