Leonardo DiCaprio has urged the Mexican president to do more in his efforts to save the critically endangered vaquita, also known as the "sea panda".
The Oscar winner also encouraged his 17.5 million Twitter followers to back a World Wildlife Federation (WWF) petition calling on Enrique Pena Nieto to take permanent measures to protect the species.
The vaquita is a small porpoise with distinctive panda-like black rings about its eyes that is only found in the upper part of Mexico's Gulf of California.
It is on the verge of extinction with fewer than 30 believed to be in existence – a population fall of 90% since 2011.
"The vaquita is the most endangered marine mammal in the world. Join me and the WWF and take action," DiCaprio tweeted, with a link to the petition addressed to Pena Nieto.
That letter calls on the Mexican premier to ban permanently the use of gill nets by fisherman in the Gulf waters. Gill nets are vertical nets that, as their name suggests, catch marine species by the gills as they attempt to pass through them.
"With fewer than 30 vaquita left, the time to act is now. #SavetheVaquita," DiCaprio said in another tweet.
Vaquita are not directly targeted by fisherman but are captured in gill nets laid for another endangered species, the totoaba fish.
The totoaba is hunted for its bladder, which is hugely popular in Chinese cuisine. It is sometimes known as "aquatic cocaine" because it can fetch up to £6,500 ($8,377) per kg.
Nieto said his government has taken steps to save the vaquita, including a two-year ban on gill net fishing, which came into effect on 31 May 2015.
He said on Twitter: "I welcome Leornado DiCaprio and the WWF's concern regarding the Vaquita Marina."
The WWF is urging him to make that ban permanent before it expires at the end of this month.