Mountainbiken

although i've never used them, i've only heard good things about Iguana Mama - Mountain Biking, Hiking & Ecological Adventures

I've hired a bike from Iguana Mama when I was in Cabarete last year, and I have to say they're a very professional company with good mountainbikes. The hilly area behind Cabarete is scenic and beautiful, although not the most spectacular of the country. Make sure you arrange a tour beforehand if you want a guided tour, because they're not so flexible they can start a tour immediately if you just turn up without notice. I made a tour on my own, so can't say anything about the quality of their guides, but I'm sure you'll have a great time.

Iguana Mama in Cabarete. 809 571-0908.

mountainbiking

I heard that Alftours in Sosua and Cabarete are doing also a mountain bike tour now. I think they start in the mountains of Moca. They do this with small groups, about 4-6 people. Maybe if you email to Alftours, you will get more information about this tour. alftour@hotmail.com

Iguana Mama has been running mountain bike tours on the North Coast for 11 years. They also won an award for best mountain biking tour in the Caribbean from Outside Mag about two years ago.
They have atleast 8 different tours ranging form beginner to expert. Their bikes are Specialized rock hopper and stump jumpers.
Iguana Mama - Mountain Biking, Hiking & Ecological Adventures

Also have a look at you-tube and find the mountainbike tour of down hill river experience, very nice......