<p><strong>Abstract.</strong> The world&#8217;s oceans are considered to be a minor source of methane (CH<sub>4</sub>) to the atmosphere although the magnitude of total net emissions is highly uncertain. In recent years the origin of the frequently observed in situ CH<sub>4</sub> production in the ocean mixed layer has received much attention. Marine algae might contribute to the observed CH<sub>4</sub> oversaturation in oxic waters, but so far direct evidence for CH<sub>4</sub> production by marine algae has only been provided for the coccolithophore <i>Emiliania huxleyi</i>. In the present study we investigated, next to <i>Emiliania huxleyi</i>, other widespread haptophytes, i.e. <i>Phaeocystis globosa</i> and <i>Chrysochromulina sp.</i> for CH<sub>4</sub> formation. Our results of CH<sub>4</sub> production and stable carbon isotope measurements provide unambiguous evidence that all three investigated marine algae produce CH<sub>4</sub> per se under oxic conditions and at rates ranging from 1.6&#8201;&#177;&#8201;0.5 to 2.7&#8201;&#177;&#8201;0.7&#8201;&#181;g&#8201;CH<sub>4</sub> per g POC (particulate organic carbon)&#8201;d<sup>&#8722;1</sup> at a temperature of 20&#8201;&#176;C with <i>Chrysochromulina sp.</i> and <i>E. huxleyi</i> showing the lowest and highest rates, respectively. In cultures that were treated with <sup>13</sup>C-labelled hydrogen carbonate &#948;<sup>13</sup>CH<sub>4</sub> values increased with incubation time, clearly resulting from the conversion of <sup>13</sup>C-hydrogen carbonate to <sup>13</sup>CH<sub>4</sub>. The addition of <sup>13</sup>C labelled dimethyl sulfide, dimethyl sulfoxide and methionine sulfoxide &#8211; known algal metabolites that are ubiquitous in marine surface layers - enabled us to clearly monitor the occurrence of <sup>13</sup>C-enriched CH<sub>4</sub> in cultures of <i>Emiliania huxleyi</i> clearly indicating that methylated sulphur compounds are also precursors of CH<sub>4</sub>. We propose that CH<sub>4</sub> production could be a common process among marine haptophytes likely contributing to CH<sub>4</sub> oversaturation in oxic waters.</p>