Abstract. We present an active surface model designed for the segmentation of Drosophila Schneider cell nuclei and nucleoli from wide-field microscopic data. The imaging technique as well as the biological application impose some major challenges to the segmentation. On the one hand, we have to deal with strong blurring of the 3D data, especially in z-direction. On the other hand, concerning the biological application, we have to deal with non-closed object boundaries and touching objects. To cope with these problems, we have designed a fully 3D active surface model. Our model prefers roundish object shapes and especially imposes roughly spherical surfaces where there is little gradient information. We have adapted an external force field for this model, which is based on gradient vector flow (GVF) and has a much larger capture range than standard GVF force fields.