Dr. Martin "Marty" Guitierrez is an American biologist living in Costa Rica who played minor yet important supporting roles in both novels. He likes to wear tweed clothing or Khaki and has a beard. He is arguably Richard Levine's best friend, Dr. Cruz is also a friend of him.
Dr. Guitierrez is summoned by Dr. Cruz to Puntarenas. His patient Tina Bowman is attacked by a lizard (a Procompsognathus) and Dr. Cruz want Dr. Guitierrez to identify it. From the description and a drawing Guitierrez concludes that the lizard has to be a Basiliscus amoratus.
However, after Cruz reports that Tina gave more specific information about the lizard (a long neck and three toes), Guitierrez revises his initial identification. He then thinks this lizard is perhaps a new species, searching for a new habitat. He goes to the beach were Tina was attacked. On the beach he finds a half-chewed lizard with that fits Tina's description.
He sends the specimen to the Laboratory for Tropical Diseases of the Columbia University to be investigated by Dr. Simpson. Dr. Simpson is absent and the scientists just take over the 'classification' that Guitierez gave to the specimen: a B. amoratus with a genetic anomaly. When Guitierez gets the report of the lab, he thinks his first classification was right. On the contrary, he felt his view of a new lizard species may also have been correct.
After the Isla Nublar Incident Guitierrez visits Alan Grant when the latter is detained in a hotel in San Jose because the authorities think he was somehow involved with Jurassic Park. He tells Grant about large lizard-like creatures in the Costa Rican jungle, and offers his opinion that Tim and Lex Murphy will probably be allowed to go home.
- "Guitierrez pushed up from his chair. He waved to Tim and Lex, playing in the pool. 'Probably they will send the children home,' he said. 'There is no reason not to do that.' He put on his sunglasses. 'Enjoy your stay with us, Dr. Grant. It is a lovely country here.'
- Grant said, 'You’re telling me we’re not going anywhere?'
- 'None of us is going anywhere, Dr. Grant,' Guitierrez said, smiling. And then he turned, and walked back toward the entrance of the hotel."
- --The final words of Jurassic Park
Guitierrez has his old position back and goes with Levine to the place where the "Abberant Form" (possibly an Ornitholestes) is first found after a two year absence.