Direct Volume Rendering (DVR) is a powerful technique for visualizing volumetric data sets. However, it involves intensive computations. In addition, most of the volumetric data sets consist of large number of 3D sampling points. Therefore, visualization of such data sets also requires large computer memory space. Hence, DVR is a good candidate for parallelization on distributed-memory multicomputers. In this work, image-space parallelization of Raycasting based DVR for unstructured grids on distributed-memory multicomputers is presented and discussed. In order to visualize unstructured volumetric datasets where grid points of the dataset are irregularly distributed over the 3D space, the underlying algorithms should resolve the point location and view sort problems of the 3D grid points. In this paper, these problems are solved using a Scanline Z-buffer based algorithm. Two image space subdivision heuristics, namely horizontal and recursive rectangular subdivision heuristics, are utilized to distribute the computations evenly among the processors in the rendering phase. The horizontal subdivision algorithm divides the image space into horizontal bands composed of consecutive scanlines. In the recursive subdivision algorithm, the image space is divided into rectangular subregions recursively. The experimental performance evaluation of the horizontal and recursive subdivision algorithms on an IBM SP2 system are presented and discussed. © Springer-Verlag Berlin Heidelberg 1996.