A dramatic reorganization of cytoplasm occurs during the first cell cycle in embryos of the nematode, Caenorhabditis elegans. We present here the results of a quantitative study of some of the events during this reorganization in wild-type embryos and in par mutant embryos. The par mutations define a set of genes required for cytoplasmic localization in early embryos. We show that par mutations lead to defects in several events of the reorganization. Mutations in all four of the par genes we studied lead to defects in pseudocleavage and asymmetric redistribution of cortical microfilaments. In addition, some of the par mutations affect streaming of cytoplasm, migration of the pronuclei, and asymmetric shortening of the embryo. We propose that the major function of the par genes might be to orchestrate this initial reorganization of cytoplasm.