Three C31 methylhopanes have been prepared by partial synthesis from appropriate diplopterol precursors. 2α-Methyldiplopterol (prepared from22-hydroxyhopan-3-one), 2β-methyldiplopterol (isolated from Methylobacterium organophilum and a mixture of diplopterol and 3β-methyldiplopterol (isolated from Methylococcus capsulatus were each converted to the corresponding 17α(H), 21β(H)-hopane. Comparison of these standards, using gas chromatography-mass spectrometry with multiple reaction monitoring, with the hopanoids from a variety of bitumens showed that all three C31 hydrocarbons may occur in sediments and that they are members of C28 and C30-C36 pseudohomologous series. 2α-Methyl-17α(H), 21β(H)-hopane and 3β-methyl-17α(H), 21β(H)-hopane are most commonly encountered in mature bitumens. 2β-Methyl-17α(H), 21β(H)-hopane occurs in some immature bitumens, is much less abundant in others of intermediate maturity and appears to be absent from mature samples. This and the similarity of the distribution patterns of homohopane and methylhomohopane isomers, indicates that the common sedimentary methylhopanes are probably derived from biogenic precursors via diagenetic processes analogous to those which give rise to hopanes. In the case of the 2α-methyl series, common to petroleum and mature sediments, derivation from the 2β-methyl hopanoids found in certain bacteria implies a maturity-related change in the configuration at C-2.