
C# 4, 58 bytes
Stream (69; 65 if weakly typed to IEnumerable)
(Assuming a using directive for System.Collections.Generic.)
IEnumerable<int>F(){int c=0,n=1;for(;;){yield return c;n+=c;c=n-c;}}
Single value (58)
int F(uint n,int x=0,int y=1){return n<1?x:F(n-1,y,x+y);}