Yolen was a Denobulan geologist that specialized in speleothems. He started his studies in 2138.

In late 2152, Yolen was working for the Denobulan Science Academy when he finally received permission to go to Xantoras to study the unique rock formations there. He led a team which included Doctors Trevix and Zepht. The three of them climbed through the maze-like tunnels to get to where they set up their base, and left sample containers throughout the caves.

On February of 2153, a xenophobic rebel faction overtook the Xantoras government and demanded that all aliens leave the planet. The scientists were too far underground to receive the message, and as the nearest Denobulan starship was several days away, the Denobulan government asked Enterprise NX-01 to retrieve him.

He was alright with riding on an alien ship back to Denobula with an Antaran, whom his people held many prejudices against. (ENT: "The Breach")

Yolen was played by actor Mark Chaet.
In the script of "The Breach", Yolen was described as considerably old and went on to say, "He's a bit eccentric, in an absent-minded-professor sort of way." (Star Trek Magazine issue 166, p. 60)
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