The North Sea Streptomyces strain GWS-BW-H5 was investigated by analyzing headspace extracts of agar-plate cultures (HE) or liquid cultures (LCE), obtained with a closed-loop stripping apparatus (CLSA), by GC/MS (Table 1). The volatile profile of the HE is dominated by the known volatiles (-)-geosmin (4) and 2-methyisoborneol (1). Small amounts of sesquiterpenes occur, which are present in a more-diverse structural variety and in higher quantities in the LCE. The different structures can be rationalized by few cationic intermediates along their biosynthetic pathway. The most-prominent difference between the two culture methods were the presence of the Me-branched gamma- and delta-lactones 31-38, not previously reported from nature, in the LCE. Major components were 10-methyldodecan-5-olide (34), 10-methyldodec-2-en-4-olide (36), and 10-methyldodec-3-en-4-olide (38). The structures of all new lactones were verified by synthesis. Furthermore, more volatiles in higher amounts were produced by the liquid culture than by to the agar plate culture. Since 36 showed inhibitory growth effects against strain GWS-BW-H5, growth inhibition against twelve other strains isolated from the same habitat was tested. Antagonistic activity against four of the strains was observed, with a slightly higher threshold level than found for penicillin G, which was used in control experiments (Table 2).