The Crab-eating Macaque (Macaca fascicularis) is an arboreal macaque native to South-East Asia. It is also called the Cynomolgus Monkey or Long-tailed Macaque.

It is used extensively in medical experiments, in particular those connected with neuroscience. It has also been identified as a possible vector for monkeypox. It is one of the types of monkeys that have been flown into space.

It is found throughout most of South-East Asia, including the islands of Sumatra, Java, and Borneo, as well as in the country of the Philippines. These monkeys are often unafraid of humans, and are found in many cities and villages. A population of Crab-eating Macaques, fed by locals, lives in the middle of the city of Lopburi in Thailand.

Crab-eating Macaques are born with black fur, but the fur turns to a yellow-green, grey-green, or reddish-brown shade as they grow. They have dark snouts, and bluish abdominal skin. They live in groups of between 20 to 60 individuals. Generally there are about 2.5 adult females for every adult male. They live for about four years in the wild, but in captivity have been known to live for up to 38 years.

There is some significant diversity within the species and these differences are classified into 10 subspecies:

• Macaca fascicularis fascicularis
• Macaca fascicularis aurea
• Macaca fascicularis umbrosa
• Macaca fascicularis atriceps
• Macaca fascicularis condorensis
• Macaca fascicularis fusca
• Macaca fascicularis lasiae
• Macaca fascicularis tua
• Macaca fascicularis karimondjawae
• Macaca fascicularis philippinensis