How do I reference types or modules defined in other F# files?

Question

I have the following in a .fs file:

namespace fsHoldem

module holdemPlayer

type Player()=
  ... etc...

and I'm trying to reference the type Player in another file. I've tried using open:

namespace fsHoldem

module BetStrategies

open fsHoldem.holdemPlayer       

but I'm getting the error "Namespace or module 'fsHoldem' is not defined."

Note that both .fs files share the same namespace, and both exist in the same project.