The production of avermectin-type antibiotics and the odours of a strain of Streptomyces avermitilis was investigated. With the increasing production of the total of avermectins (0-600 mu g/ml), the synthesis of geosmin is also enhanced by more than one order of magnitude and, in contrast, the production of homologues of oxolone (dihydrofuranone) decreases in a similar range. In addition to these compounds, more than twenty other volatile chemical products have been identified by means GC-MS, i.e. pyrazine derivatives, acetoin and its homologues, aromatic esters, furan derivatives, etc.).