Hi all, with PETSc v3.9 there seems to be a out of bounds write in the fortran interface of MatCreateSubmatrices. https://bitbucket.org/petsc/petsc/src/59479656cffe26dd9a9d446f6d00dfe5e31681... The reference mentions that in fortran one should pass a preallocated array for submat with at least the size of m (actually it's probably a typo and should be n) http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateSub... Then n+1 entries are copied into that array, where the last entry seems to be some unneeded dummy (as mentioned). for (i=0; i<=*n; i++) { /* lsmat[*n] might be a dummy matrix for saving data struc */ smat[i] = lsmat[i]; } This leads to some bad errors afterwards, when that array is only of the size of n as recommended. Thanks and kind regards, Volker