Abstract. In this paper, the symplectic genus for any 2−dimensional class in a 4−manifold admitting a symplectic structure is introduced, and its relation with the minimal genus is studied. It is used to describe which classes in rational and irrational ruled manifolds are realized by connected symplectic surfaces. In particular, we completely determine which classes with square at least −1 in such manifolds can be represented by embedded spheres. Moreover, based on a new characterization of the action of the diffeomorphism group on the intersection forms of a rational manifold, we are able to determine the orbits of the diffeomorphism group on the set of classes represented by embedded spheres of square at least −1 in any 4−manifold admitting a symplectic structure. §1 Introduction Let M be a smooth, closed oriented 4−manifold. An orientation-compatible symplectic form on M is a closed two−form ω such that ω ∧ ω is nowhere vanishing and agrees with the orientation. For any oriented 4−manifold M , its symplectic cone C M is defined as the set of cohomology classes which are represented by orientationcompatible symplectic forms. For any class e ∈ H 2 (M ; Z), its minimal genus m(e) is the minimal genus of a smoothly embedded connected surface representing the Poincaré dual PD(e). The problem of determining the minimal genus has involved many of the important techniques in 4−manifold topology, and it bears its origin in the older problem of representing the Poincaré dual to a class by an embedded sphere (See the excellent survey papers [La1-2] and [Kr1] on these two problems).We are here interested in studying both these problems for 4−manifolds with non-empty symplectic cone. We will introduce the notion of symplectic genus η(e) for 4−manifolds with non-empty symplectic cone. Recall that any symplectic structure ω determines a homotopy class of compatible almost complex structures on the cotangent bundle, whose first Chern class is called the canonical class of ω. Roughly, the symplectic genus η(e) of a class e is given by the formula [e 2 +K · e]/2 +1, where K has largest pairing against e amongst canonical classes of symplectic structures for which the symplectic area of e is positive.η(e) has many nice properties, among which are (i) invariance under the action 1