<p>Selected microscopic fungi from extreme marine ecosystems have unique capacities to degrade complex oil molecules, which confers them a growing interest in bioremediation. This study evaluated the oil-degrading capabilities of six fungal isolates from deep-sea hydrothermal vents. The response of the isolates to the presence of light crude oil (LCO) and heavy crude oil (HCO) was assessed through a tolerance bioassay, while their capabilities to degrade the oil as the sole carbon source were tested in a biodegradation bioassay. The assimilation of oil derivatives into fungal tissues was determined by their carbon (<i>δ</i>¹³C) and nitrogen (<i>δ</i>¹⁵N) isotopes. The compositional changes in HCO after exposure to <i>Aspergillus terreus</i> and <i>Penicillium miczynskii</i> (isolate F) were assessed. All isolates grew in the presence of both oils in the tolerance bioassays, yet <i>A. terreus</i>, <i>Aspergillus sydowii</i>, and <i>P. miczynskii</i> (isolates E and F) showed enhanced growth when using crude oil as the sole carbon source. The <i>δ</i>¹³C was more enriched in the isolates than in the oil, suggesting they used it as a carbon source. The analysis of the oil exposed to fungal activity showed that fungi degraded medium and long-chain hydrocarbons. Particularily,&#xa0;<i>P. miczynskii</i> (F) showed a&#xa0;remarkable growth in the bioassays and the ability to degrade complex hydrocarbons, representing a promising bioremediation agent.</p>