Fertilization overcomes meiotic arrest by triggering a series of biochemical events, resulting in activation of the egg. A small group of protein tyrosine kinases (PTKs) have been identified in eggs of invertebrates and lower vertebrates and inhibitor studies have suggested that they play a role in late events of egg activation. A recent study using the sea urchin system demonstrated that Fyn kinase was expressed in eggs and was activated within minutes of fertilization. In the present study, Western blot analysis as well as immune complex kinase assay demonstrated that p59(c-fyn) kinase was expressed in both unfertilized and fertilized rat eggs. Immunofluorescence confocal microscopy demonstrated that Fyn kinase was localized to the egg cortex but also to the polar body and the fertilizing cone which are elevated from the cortical cytoplasm of the activated egg. Surprisingly, Fyn was also found to be highly concentrated over the meiotic and mitotic spindles. To date, Fyn is the first PTK demonstrated to be present in the mammalian egg. Localization of Fyn to the egg cortex as well as the spindle microtubules indicates that this protein kinase may have multiple functions within the egg.