Abstract. Let X be a connected open Riemann surface. Let Y be an Oka domain in the smooth locus of an analytic subvariety of C n , n ≥ 1, such that the convex hull of Y is all of C n . Let O * (X, Y ) be the space of nondegenerate holomorphic maps X → Y . Take a holomorphic 1-form θ on X, not identically zero, and let π : O * (X, Y ) → H 1 (X, C n ) send a map g to the cohomology class of gθ. Our main theorem states that π is a Serre fibration. This result subsumes the 1971 theorem of Kusunoki and Sainouchi that both the periods and the divisor of a holomorphic form on X can be prescribed arbitrarily. It also subsumes two parametric h-principles in minimal surface theory proved by Forstnerič and Lárusson in 2016.We start by recalling three theorems from the early decades of modern Riemann surface theory. Behnke and Stein proved that the periods of a holomorphic form on an open Riemann surface X can be prescribed arbitrarily [3, Satz 10] (see also [5, Theorem 28.6]). In other words, every class in the cohomology group H 1 (X, C) contains a holomorphic form. Gunning and Narasimhan showed that the zero class contains a holomorphic form with no zeros [9]. In other words, there is a holomorphic immersion X → C. Kusunoki and Sainouchi generalised these two theorems and proved that both the periods and the divisor of a holomorphic form on X can be prescribed arbitrarily [10, Theorem 1].Our main result subsumes the theorem of Kusunoki and Sainouchi as a very special case. It also subsumes different and much more recent results from the theory of minimal surfaces, which we shall now describe. Let M * (X, R n ) denote the space (with the compact-open topology) of conformal minimal immersions X → R n , n ≥ 3, that are nonflat in the sense that the image of X is not contained in any affine 2-plane in R n . Some such immersions are obtained as the real parts of holomorphic null curves, that is, holomorphic immersions X → C n directed by the null quadric A = {(z 1 , . . . , z n ) ∈ C n : z 2 1 + · · · + z 2 n = 0}. We denote by N * (X, C n ) the space of holomorphic null curves that are nonflat, meaning that the image of X is not contained in any affine complex line in C n .