During early stages of meiotic prophase I the nucleus undergoes considerable reorganization, including the clustering of telomeres, the release of contacts between chromosomes and the nuclear membrane, the reorganization of the nucleolus, and chromatin remodeling. Using a light squashing technique for the analysis of meiotic chromosomes along with fluorescent in situ hybridization, transmission electron microscopy and immunolocalization studies with antibodies to modified histones, we demonstrate that ASK1 is essential for early nuclear reorganization events. A relatively large number of meiotic alterations have been identified in ask1-1 plants. We show that many of these defects, including alterations in homolog pairing, nucleolus migration and the missegregation of chromosomes, may arise from alterations in chromatin structure and the inability of chromosomes to resolve and release properly from the nuclear membrane and nucleolus during leptotene. These results raise the interesting possibility that ASK1 controls chromatin structure by targeting of either an early regulator of meiotic progression or possibly matrix attachment proteins for destruction.