  We classify tight contact structures on the small Seifert fibered 3--manifold
M(-1; r_1, r_2, r_3) with r_i in (0,1) and r_1, r_2 \geq 1/2. The result is
obtained by combining convex surface theory with computations of contact
Ozsvath--Szabo invariants. We also show that some of the tight contact
structures on the manifolds considered are nonfillable, justifying the use of
Heegaard Floer theory.
