The distribution of microtubules and microtubule organizing centers in the events leading up to the establishment of the first asymmetric cleavage furrow in nematode embryos was followed using indirect immunofluorescence of antibodies to tubulin. Oocytes arrest in meiotic prophase then undergo two meiotic reduction divisions after fertilization. At both of these divisions barrel-shaped spindles were observed. Initially a single microtubule organizing center was seen adjacent to the sperm pronucleus following fertilization in Caenorhabditis elegans, but later two sperm asters were distinguished. These increased in size as the egg pronucleus migrated toward the sperm pronucleus and reached maximum size, with fascicles of microtubules extending to the cortex, once the pronuclei had become juxtaposed. The first cleavage spindle formed following rotation and migration of the juxtaposed pronuclei back toward the center of the embryo. The distribution of microtubules in a temperature-sensitive mutant that fails in both pronuclear migration and rotation was also examined. Asters in the mutant embryos at the nonpermissive temperature contained only short microtubules suggesting that the morphology of the asters is important for directing the movement of the pronuclei. In Panagrellus redivivus sperm asters were not detected by anti-tubulin staining until the female pronucleus had migrated to the centrally placed sperm pronucleus. Asters then increased in size and formed the first cleavage spindle.