<p><strong>Abstract.</strong> Sei whales (<i>Balaenoptera borealis</i>) is the least known whale species. Information on sei whale´s vocalizations in the south-eastern Pacific Ocean and its regional variability are even more scarce than that from other ocean areas. This research presents the first characterization of sei whale sounds recorded in Chile during austral autumn of 2016 and 2017. A total of 41 calls have been attributed to sei whale's downsweeps. In 2016, calls ranged from an average

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... maximum frequency of 105.3<span class="thinspace"></span>Hz down to an average minimum 35.6<span class="thinspace"></span>Hz over 1.6<span class="thinspace"></span>s with a peak frequency of 65.4<span class="thinspace"></span>Hz. During 2017, calls ranged from an average maximum frequency of 93.3<span class="thinspace"></span>Hz down to 42.2<span class="thinspace"></span>Hz (over 1.6<span class="thinspace"></span>s) with a peak frequency of 68.3<span class="thinspace"></span>Hz. The absolute minimum frequency recorded was 30<span class="thinspace"></span>Hz and the absolute maximum frequency was 129.4<span class="thinspace"></span>Hz. Calls generally occurred in pairs, but triplets or singles were also registered. These low frequency sounds share characteristics with recordings of sei whales near the Hawaii Islands, but with differences in the maximum frequencies and duration. These calls distinctly differ from sounds previously described for sei whales in the Southern Ocean and are the first documented sei whale calls in the South-eastern Pacific.</p>