A Costa Rican seep oil known to have a high isopranolu content was analyzed for the presence and relative amounts of branched alkanes, aspadally isoprenoid alkanes in the range of C16-C30. The analysis was conducted by means of gas chromatography and mass spectrometry and is an effort to gain an insight into possible precursor cr precursors cf the isoprencicis. The iseprenoid content of various geological samples can possibly then be used to indicate the presence of living organisms and, the diagenetic mechanisms which alter buried organic matter. The oil was fractionally distilled, and gas chromatography used to separate and purify the constituent compounds. Approximate measurements of the relative amounts of the constituents, and rough correlations of unknowns with standard alkanes were also obtained by gas chromatography. The purified samples ware than analyzed by low-resolution mass spectrometry. The analysis revealed a complete set of regular isoprenoids from C16 to C25, except for the C17 and C22 regular isoprenoids. The largest component was the C19 isoprenoid (pristane) and the second largest component was the C20 isoprenoid (phytane). Though the C17 regular isoprenoid was absent, three-, seven-, sieven-trimethyl tetradecane was tentatively Identified and the possible presence of three-, seven-, eleven-triniethyl hoxadacane noted. There was no evidence of irregular branched isobars that would arise from squalane degradation, and the evidence strongly Indicates phytol, rather than squalane, as the major precursor for isoprenoids below C21. The C23, C24, and C25 regular isoprenoids were also identified. These have been rarely reported by other researchers, and the amount present was considerably less than that of the isoprenoids. No C22 regular isoprenoic, and no irregularly branched C24 and C25 isomers ware found, suggesting-that a regular isoprenoid structure, rather than lycopene or a similar compound, is the precursor. Squalane was not found. The C28 and C30 regular isoprenoids ware tentatively identified, by mass spectrometry. The existence of these two compounds suggest possible precursors such as regular isooranyl alcohols of the C3Q size or greater.