In both invertebrate and vertebrate systems, the localization of maternal mRNAs is a common mechanism used to influence developmental processes, including the establishment of the dorsal/ventral axis, anterior/posterior axis, and the germ line (for review, see Bashirullah et al., 1998. Annu. Rev. Biochem. 67, 335-394). While the existence of localized maternal mRNAs has been reported in the zebrafish, Danio rerio, the precise localization patterns of these molecules during oogenesis has not been determined. In this study, in situ hybridization experiments were performed on zebrafish ovaries and activated eggs to examine different mRNA localization patterns. The results establish that while some maternal mRNAs remain ubiquitously distributed throughout the oocyte, other mRNAs follow specific localization patterns, including localization to the animal pole, localization to the vegetal pole, and cortical localization. The animal/vegetal axis is first apparent in stage II oocytes when the earliest mRNA localization is seen. Unique patterns of localization are seen in mature eggs as well. Some mRNAs maintain their oocyte localization patterns, while others localize upon egg activation (fertilization).